feat: migrate whole document in a single pass via custom FixAllProvid…

…er (#22)

The previous BatchFixer-based pipeline dropped per-diagnostic rewrites whose text changes overlapped on the using-directive line, and once the first using swap landed the analyzer no longer fired on remaining TestableIO bindings — leaving files partially migrated and uncompilable.

Each per-pattern Apply method is split into a pure helper that operates on (CompilationUnitSyntax, SyntaxNode) without touching usings. A new SystemIOAbstractionsFixAllProvider annotates every diagnostic target, dispatches the pure helpers sequentially with annotation-relocation and re-acquired semantic models, then applies a single using-directive change at the end. The per-diagnostic CodeAction re-runs the analyzer to find sibling diagnostics in the same document and routes through the same provider, so one click in the IDE migrates the whole file (Mockolate-style).

docs: Wire HelpLinkUri and expand migration documentation (#18)

This pull request introduces documentation and usability improvements for the migration analyzer package, clarifies installation and workflow, and updates links to the new documentation site. The most important changes are grouped below.

**Documentation and User Guidance Improvements:**

* Expanded the `README.md` with a detailed migration workflow, explicit instructions for referencing both the migration and target packages, and tables outlining supported and manual-review migration patterns. Added sections on suppressing diagnostics and clarified that the migration package is a development-only tool.
* Added a new documentation page (`00-index.md`) for the migration package, including a NuGet badge and a summary of its purpose.

**Links and Metadata Updates:**

* Updated the documentation and package URLs to use the new path (`Abstractions/Migration`) instead of the old one (`Testably.Abstractions/Migration`) in both MSBuild properties and analyzer help links.

**CI/CD Workflow Maintenance:**

* Updated the GitHub Actions workflow to use `peter-evans/repository-dispatch@v4` for dispatching documentation update events, ensuring continued compatibility and support.