Add documentation snippets

[martincostello]

• Use mdsnippets to get code snippets in markdown from compile-validated code.
• Use badges for dependency versions in README.
• Add dependency of Microsoft.OpenApi to the README.

Cherry-picked from #3283.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.28%. Comparing base (549e65b) to head (81d5870).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3592   +/-   ##
=======================================
  Coverage   94.28%   94.28%           
=======================================
  Files         110      110           
  Lines        3816     3816           
  Branches      723      723           
=======================================
  Hits         3598     3598           
  Misses        218      218           

Flag	Coverage Δ
Linux	94.28% <ø> (ø)
Windows	94.28% <ø> (ø)
macOS	94.28% <ø> (ø)

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[Copilot]

Pull Request Overview

This PR introduces mdsnippets to extract code snippets from compiled C# code into documentation. It adds compile-validated code examples to markdown documentation, uses version badges for dependencies in the README, and creates a test project containing all the code snippets used throughout the documentation.

• Add mdsnippets tooling to generate compile-validated code snippets in markdown
• Create DocumentationSnippets test project with all example code
• Update README and documentation files with snippet references and dependency version badges

Reviewed Changes

Copilot reviewed 42 out of 42 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/WebSites/DocumentationSnippets/	New test project containing all code examples used in documentation
mdsnippets.json	Configuration file for mdsnippets tool
README.md	Updated with snippet references and dependency version badges
docs/*.md	Updated documentation files with mdsnippets references
.github/workflows/update-documentation.yml	New workflow to automatically update documentation snippets
.config/dotnet-tools.json	Added mdsnippets tool configuration

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