Clean Up Legacy AutoRest Documentation Under documentation

### Background
The Azure REST API Specs repository has transitioned toward TypeSpec as the primary authoring experience. However, the documentation folder still contains guidance that references legacy AutoRest-based workflows, tooling, configurations, and processes.

### Actions

- Review documents under documentation for legacy AutoRest references.
- Remove obsolete content that no longer provides value.
- Add deprecation notices where historical guidance should remain available for reference.
- Update links and references to point to TypeSpec-based workflows and tooling.
- Ensure service teams can easily identify the recommended authoring and validation experience.

Out of Scope
- Functional changes to TypeSpec tooling.
- SDK generation pipeline changes.
- Changes outside the documentation directory unless required to fix broken references.

Start by reviewing the documentation directory and searching for legacy AutoRest references, workflows, tooling, configurations, and processes. Identify which guidance is obsolete, which needs a deprecation notice, and which links should point to TypeSpec workflows. Done means the documentation clearly identifies the recommended authoring and validation experience without changing TypeSpec tooling or SDK pipelines.

Written by the indexing model from the issue text.