docs(standards): document REVIEW.md as a required file

[sbaerlocher]

Summary

• Adds REVIEW.md to the required-files list and the recommended structure tree in standards.rst; it exists in seven arillso repositories with an identical five-heading structure but was documented nowhere.
• Documents what the file is for: ai-claude-review.yml in arillso/.github reads it on every pull request and treats it as taking precedence over generic best practices.
• Documents the base-branch rule — the workflow reads the file from .review-base/ and ignores the copy at the repository root, so a pull request cannot rewrite the rules of its own review.
• Specifies the five headings as the shared contract, with the severity levels table and guidance on writing the two repository-specific sections.

Test plan

• Sphinx build succeeds (./build.sh) — not runnable in the authoring environment, left to CI
• :ref: targets resolve: review-md is defined once and referenced from the workflow naming section and the seealso block

[claude]

No issues found.

Reviewed the RST changes in rst/guide/best-practices/standards.rst:

• Section underlines all meet/exceed title length (verified char counts).
• :ref: label review-md defined once and referenced twice — no duplicate/broken refs.
• Enumerated list sequential (1–10); file-tree entry correctly placed.
• list-table, .. important::, and code-block directives well-formed.
• Documented severity levels and five-heading contract are consistent with this repo's own REVIEW.md.

Build success and ref resolution are correctly deferred to CI.