Add check ReadmeValidation

[mikeharder]

• Pipeline uses GH Action
• Tool is clone of TypeSpecValidation
• Rules
  • Verify default $(tag) exists
  • Add more rules we can think of now, or as we find more bugs in specs
• Support suppressions.yaml
• Run on PRs to give clear guidance how to fix readme.md
• Run on all specs in main
  • If only a few failures, add suppressions then enable pipeline "ReadmeValidation - All"

[konrad-jamrozik]

Avocado was in effect our "README validation" check before. There is bunch of work related to it:

• https://github.com/Azure/azure-sdk-tools/issues?q=is%3Aopen+is%3Aissue+Avocado
• https://github.com/Azure/avocado/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc

The README validation ideally should build directly on this work (once ready):

• Develop semantic model of specs repo dir structure azure-sdk-tools#8203

For suppressions, we probably should also support the README.md suppressions and sdk-suppressions.yaml:

• Documentation: Spec PR tools suppressions mechanisms reference azure-sdk-tools#8133

[konrad-jamrozik]

I had a chat with @mikeharder. We are considering introducing a check named Uniform Versioning Validation. It would take care of directory structure validation and README.md validation, ensuring, among others, that they follow the uniform versioning principles as outlined in this PR:

• Update directory-structure.md: clarify service, service group, uniform versioning. #28929