Define six-library documentation quality standard

[stefan-jansen]

Summary

• Define a common reader and content standard for the six user-facing ML4T library sites.
• Record a dated inventory and quality review, including Backtest's verified documentation checks and current book-path drift elsewhere.
• Remove obsolete ignored review notes from the MkDocs source tree after preserving them in local work-unit history.

Evidence

• uv run mkdocs build --strict passed in the ecosystem repository and all six library checkouts on 2026-09-24.
• Backtest's book-link, rendered-link, and selected installed-wheel example checks passed. The dated review states the scope and limits.
• Six library-specific prompts are committed and verified on the private development sidecar remotes.

Scope

This PR sets the shared standard and baseline. Library documentation changes belong in each public library's issue and PR. The broader coordination issue remains open for the six follow-up reviews and successor guidance.

Part of #43.