Add a documentation learning path and fix issues found while verifying it (#4555)

## Summary

### Documentation
- Stage the documentation index as a learning path: a first application,
independent client, server, and PubSub paths, then data and services,
companion models, production, advanced integration, and draft or
experimental capabilities.
- Add Getting started (repository samples or NuGet packages), OPC UA
concepts, Your first information model, and a production readiness
checklist.
- Add Native AOT application publishing, a first PubSub publisher and
subscriber exchange, and the UDP transport options.
- Add or expand contents navigation throughout `docs/`, and keep
contributor documentation separate.
- Correct outdated SDK examples and capability guidance, consolidate
repeated explanations into canonical guides, and repair local links and
heading structure.
- Rewrite What's New in 2.0 as a concise feature overview with short
linked details; keep migration instructions in the Migration Guide.
- Expand the feature catalogue with specification IDs, GDS, LDS, the OPC
10000-21 onboarding registrar, ISA-95, xRegistry, OpenUSD, and
sample-local models; distinguish draft status, explain crypto
pluggability, and bring performance guidance forward.
- Address review feedback on wording, active monitored items during
shadow reload, and the `Samples.md` filename and references.
- Regenerate the `opcua-v20-migration` plugin's bundled copy of the
migration guides so it matches the edited `docs/migrate/2.0.x` guides.
- Merge the latest `master`, preserving upstream resource-isolation
guidance within the task-based documentation index.
- Retain deferred Lingity findings where appropriate; this PR does not
claim that every document meets the Lingity score thresholds.

### Fixes found while verifying the guides
Each fix has regression tests that fail without it.
- **Client:** V2 subscriptions compared their lifetime with the
operation timeout instead of the session timeout, and logged a spurious
warning. `ISubscriptionEngineContext`, which is new in 2.0, gains a
`SessionTimeout` property; implementations must add it.
- **PubSub:** a subscriber now registers the `DataSetMetaData`
configured on its readers (OPC 10000-14 6.2.9.4), and the UADP decoder
decodes DataSetMessages that carry no ConfigurationVersion. Before, a
secured subscriber of the reference sample received no DataSets.
- **Source generator:** a ModelDesign method that declares its arguments
inline, without a method type, produced code that did not compile; it
now uses `MethodState`. The generated code of existing models is
unchanged.
- **MinimalClient sample:** waits for its first data-change notification
before it disconnects.

### Known issues, not addressed here
- UDP multicast: the publisher does not send on the interface that the
subscriber joins, and `MulticastLoopback` defaults to false. The PubSub
guide uses unicast for the exchange on one computer.
- The UDP metadata announcement is sent unsecured on secured writer
groups; a secured subscriber drops it with a warning.
- A server does not start in globalization-invariant mode, because
`ResourceManager` creates `CultureInfo("en-US")`. The Native AOT guide
cautions against the mode.

## Validation
- `git diff --check` and
`./.azurepipelines/validate-migration-plugin.ps1` (15 bundled migration
docs validated, plugin manifests synchronized).
- Local Markdown links, anchors, file-link casing, and contents lists
checked across all 87 documents under `docs/`. The index links every
top-level guide and every target of the previous index.
- Both Getting started routes, Your first information model, the PubSub
exchange, and the Native AOT publish of the Getting started client and
server were run as documented. The NuGet route and the tutorial use the
public `2.0.0-preview.5` packages.
- The changed libraries build for all target frameworks in Release
without warnings.
- Tests on .NET 10: Opc.Ua.SourceGeneration.Tests 199 passed;
Opc.Ua.SourceGeneration.Core.Tests 3,977 passed, 8 skipped;
Opc.Ua.Server.Tests `Fluent` category 621 passed; Opc.Ua.Client.Tests
subscription-engine tests 70 passed; Opc.Ua.PubSub.Tests 1,374 passed, 2
skipped; PubSub fuzz replay 145 passed. The new and affected tests also
pass on .NET Framework 4.8.
- Earlier merge validation: 174 resource-isolation tests passed on .NET
10.
- The full solution suite was not run locally.

Review and validation artifacts are maintained outside the repository.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/ModelChangeTracking.md

@@ -1,19 +1,19 @@
 # Model Change Tracking
 
-OPC UA Part 5 §6.4.32 defines `GeneralModelChangeEventType` so a
-server can notify clients that the address space has changed —
-nodes added or deleted, references added or deleted, datatypes
-changed. The `Opc.Ua.Client.ModelChange` namespace ships an
-opt-in client-side tracker that consumes those events, drops the
-affected entries from the client's `INodeCache`, and surfaces the
-changes through a strongly-typed event so application code can
-re-browse the impacted subtrees.
+OPC UA Part 5 §6.4.32 defines `GeneralModelChangeEventType`. Servers use it
+to notify clients about address-space changes, including added or deleted
+nodes and references, and changed data types.
 
-The server-side counterpart lives in `Opc.Ua.Server.Alarms`:
-`ModelChangeAggregator` batches per-call changes and the
-`CustomNodeManager` (and `AsyncCustomNodeManager`) emit
-`GeneralModelChangeEvent` automatically from `CreateNode` /
-`DeleteNode`. Opt out via a flag if you need manual control.
+The `Opc.Ua.Client.ModelChange` namespace provides an opt-in client-side
+tracker. It consumes these events, removes affected entries from the
+client's `INodeCache`, and surfaces the changes through a strongly typed
+event. Application code can then browse the affected subtrees again.
+
+The server-side counterpart lives in `Opc.Ua.Server.Alarms`.
+`ModelChangeAggregator` batches per-call changes. `CustomNodeManager` and
+`AsyncCustomNodeManager` automatically emit `GeneralModelChangeEvent` from
+`CreateNode` and `DeleteNode`. Set a flag to opt out when you need manual
+control.
 
 - [Quick reference](#quick-reference)
 - [Server side: emitting model changes](#server-side-emitting-model-changes)
@@ -70,8 +70,11 @@ public class MyNodeManager : AsyncCustomNodeManager
 ```
 
 > **Browse consistency.** `CreateNodeAsync`, `AddNodeAsync` and `DeleteNodeAsync` also keep the node manager's internal component cache in sync with the change.
-> A deleted node is evicted from the cache, and the parent's cached view is refreshed after a runtime add or remove, so a Browse, Read or Call issued afterwards reflects the committed child set instead of a stale, cached view.
-> Re-registering or replacing a node id (for example swapping a passive child for a typed proxy) refreshes the cached instance only when that node is already cached.
+> A deleted node is evicted from the cache. After a runtime add or remove, the
+manager refreshes the parent's cached view. A subsequent Browse, Read, or
+Call therefore sees the committed child set instead of stale data.
+> When the node is already cached, re-registering it refreshes the cached
+instance. Replacing a cached node with a typed proxy also refreshes it.
 
 If your node manager mutates the address space without going through
 `CreateNodeAsync` / `DeleteNodeAsync` (for example by editing an
@@ -264,10 +267,10 @@ reports the address-space change. Without a refresh, a Client resolves
 NodeIds from the new namespace against a stale `NamespaceUris` table
 and silently gets the wrong index.
 
-The tracker closes that gap. Given an `INamespaceTableRefresher` it
-re-reads the namespace table before it invalidates the cache and before
-it raises `ModelChanged`, so subscribers that re-browse from the handler
-already see the new uris:
+The tracker closes that gap when given an `INamespaceTableRefresher`. It
+refreshes the namespace table before invalidating the cache and raising
+`ModelChanged`. Subscribers can then re-browse from the event handler using
+the new URIs:
 
 ```csharp
 public interface INamespaceTableRefresher
@@ -351,7 +354,6 @@ OPC UA Part 5 §9.32.2 ties `ModelChangeEvent` emission to the
 > changed, a `ModelChangeEvent` shall be generated. A Server shall
 > support both the `ModelChangeEvent` and the `NodeVersion` Property
 > or neither, but never only one of the two mechanisms."
->
 > "Only those Nodes of the AddressSpace having a `NodeVersion` shall
 > trigger a `ModelChangeEvent`. Other Nodes shall not trigger a
 > `ModelChangeEvent`."