docs: Add guide for generating versioned docsets.

[kapunahelewong]

Adds an end-to-end guide for generating and publishing a versioned documentation
snapshot for a new Bazel release.

What's in the guide

• Two build paths: detects whether the release branch has MDX tooling
  (Path A: gen_mdx_release_docs) or needs the older pipeline plus
  docs2mdx.py (Path B), with a one-liner to check before starting
• Prerequisites: Bazelisk, GNU grep (macOS), Mintlify CLI, GitHub CLI,
  Python deps for Path B
• 9 steps end-to-end with copy-pasteable commands split by path where
  they differ
• MDX fix checklist: 6 error categories (Jekyll anchors, {: .external},
  Jinja-escaped tags, DevSite metadata, non-self-closing void elements) with
  exact sed/perl one-liners
• AI prompt for harder-to-automate fixes (unescaped </> in prose,
  <pre> blocks with special characters)
• Anchor restoration: shell snippet to find the best source version
  dynamically rather than hardcoding a version number
• Gotchas inline: UNSAFE_ version stamps, wrong base branch, double-nested
  extraction, CLA bot, command-line-reference.mdx presubmit failure
• Final checklist to run through before submitting the PR

[kapunahelewong]

Keeping this in draft mode until I can test it to confirm accuracy.