Add docs index for reviewer navigation

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Summary

• add docs/README.md to separate the current reviewer route from supporting design notes and historical release evidence
• link the docs index from the root README Repo Guide
• add reviewer-doc assertions for the docs index route and historical evidence boundary

Validation

• pytest tests/test_reviewer_docs.py tests/test_markdown_links.py --basetemp .pytest-artifacts-docs-index
• pytest --basetemp .pytest-artifacts-full-docs-index
• git diff --cached --check
• staged diff privacy/secret keyword scan