Docs: add a first-contribution workflow for docs-only contributors

What is missing today

The repo has docs/contributing.md and issue templates, which is great. A small missing piece is a single "first external contribution" path for docs contributors that tells new users exactly what to do in sequence.

Suggested improvement

Add a short section in README.md (or top of docs/contributing.md) called "First Contribution (Docs)" with:

1. how to choose a docs issue
2. local docs validation command(s)
3. expected PR structure and checklist
4. what reviewers usually look for

Why this is useful

Codex has very high visibility and issue volume; a streamlined docs-first path could reduce contributor friction and improve quality of first PRs.

If useful, I can propose a concrete patch to docs/contributing.md.