docs: add public release documentation QA checklist

[yaoyao12345666]

What does this PR do?

Adds a discoverable public-release documentation QA checklist covering:

• strict HTML and external-link validation;
• navigation, intentional orphans, and API/CLI contract drift;
• example-command freshness and troubleshooting links;
• CPU-safe versus GPU-dependent verification; and
• public-content/privacy review and final sign-off evidence.

This is documentation-only and does not change runtime behavior.

Related issue

Fixes #146

Type of change

• Documentation update

How was it tested?

• python -m sphinx -E -a -W --keep-going -b html docs .docs-build-146-final
• python -m sphinx -E -a -W --keep-going -b linkcheck docs .docs-linkcheck-146-final
• ruff check docs --no-cache
• verified areno train --help, areno serve --help, areno check --help, and areno env --help

Both strict Sphinx builds completed without warnings or broken links.

Checklist

• The PR title summarizes the contribution.
• Linked the related issue in the description.
• Existing tests pass (pytest tests/ -k cpu) - not rerun for this documentation-only change on Windows.
• New documentation passed strict HTML and link validation.
• Described the validation commands and environment limitation.
• No public API or CLI behavior changes.