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- Correct outdated SDK examples and capability guidance, consolidate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/DeveloperGuide.md

@@ -4,6 +4,32 @@ This guide is the starting point for contributing to the OPC UA .NET Standard st
 
 If you are new here, read the sections in order: [Prerequisites](#prerequisites) → [Repository layout](#repository-layout) → [Building](#building) → [Running tests](#running-tests) → [Coding standards](#coding-standards-dos-and-donts). The [How-to guides](#how-to-guides) and [Packages, platform support, and versioning](#packages-platform-support-and-versioning) sections are reference material you can jump to as needed.
 
+## Contents
+
+- [Prerequisites](#prerequisites)
+- [Repository layout](#repository-layout)
+- [Building](#building)
+- [Running tests](#running-tests)
+- [Coding standards (dos and don'ts)](#coding-standards-dos-and-donts)
+- [How-to guides](#how-to-guides)
+  - [Add a log message (source-generated)](#add-a-log-message-source-generated)
+  - [Other common tasks](#other-common-tasks)
+- [Packages, platform support, and versioning](#packages-platform-support-and-versioning)
+  - [Released packages](#released-packages)
+  - [Supported target frameworks](#supported-target-frameworks)
+  - [Supported analyzer and source generator hosts](#supported-analyzer-and-source-generator-hosts)
+  - [Versioning](#versioning)
+- [Continuous integration](#continuous-integration)
+  - [What runs where](#what-runs-where)
+  - [Test tiers](#test-tiers)
+  - [Running the full scope](#running-the-full-scope)
+  - [Required checks and coverage](#required-checks-and-coverage)
+  - [Reproducing a CI leg locally](#reproducing-a-ci-leg-locally)
+  - [Triggering a pipeline run on a pull request](#triggering-a-pipeline-run-on-a-pull-request)
+  - [Migration status](#migration-status)
+- [Contributing and pull requests](#contributing-and-pull-requests)
+- [Related documentation](#related-documentation)
+
 ## Prerequisites
 
 - **.NET SDK 10.0** — the whole repository builds and restores with the [.NET 10 SDK](https://dotnet.microsoft.com/download/dotnet/10.0). Older SDKs are not supported for building `main`. The class libraries still *target* older frameworks (see [Packages, platform support, and versioning](#packages-platform-support-and-versioning)), but you build them with the .NET 10 SDK.
@@ -277,7 +303,7 @@ The analyzer and source generator packages ship under `analyzers/dotnet/roslyn<m
 The version is declared once in `roslyn.props`.
 
 > **Adding a band below 4.14 is not just another entry in that file.** The analyzer closure — the generator, `Opc.Ua.SourceGeneration.Core` **and** `Opc.Ua.Types` — must bind against the Roslyn host's own `System.Collections.Immutable` and `System.Reflection.Metadata`. .NET satisfies a reference from a *higher* assembly version but never from a lower one, and those assemblies are supplied by the compiler, so the closure must reference the lowest version across every supported band and must never ship a copy of its own. Roslyn 4.14 and 5.0 both depend on 9.0.0, which is why `$(RoslynRuntimeVersion)` in `roslyn.props` drives the central pin and one build of the non-Roslyn closure serves both bands. Going lower — Roslyn 4.8 wants 7.x — would mean building that whole closure, `Opc.Ua.Types` included, a second time.
->
+
 > Get it wrong and the failure is silent: the generator is skipped (`CS9057`), fails to load (`CS8032`) or throws `MissingMethodException` while initializing (`CS8784`) — all *warnings*, so the consumer just gets no generated code. `validate-source-generator-packages.ps1` therefore refuses any package that ships `Microsoft.CodeAnalysis*`, `System.Collections.Immutable` or `System.Reflection.Metadata`, and runs the packed down-level payload through a real compiler of that band.
 
 ### Versioning