[paulbalandan]

Description
Part 5 of the user guide cutover in #10683, after #10714, #10715, #10716 and the switch in #10717. This is the last PR of the series. It updates the contributor docs and the tooling that still refer to user_guide_src/ and Sphinx.

Two commits:

1. Rewrite the contributor guide for Markdown.
   • contributing/documentation.rst and its TextMate bundle are replaced by contributing/documentation.md. It covers the MkDocs setup (mkdocs serve, mkdocs build --strict), the navigation in docs/.nav.yml with admin/update-docs-nav.php, headings, links, admonitions, code fences and the Python-Markdown whitespace rules.
   • contributing/pull_request.md links to the new guide, to docs/changelogs and docs/installation, and to the changelog and upgrading pages under /develop/ on GitHub Pages.
   • .github/instructions/user-guide.instructions.md now applies to docs/**/*.md.
2. Update tooling references to the Markdown user guide.
   • carson.yml labels docs/** as documentation.
   • The forum-announcement skill reads docs/changelogs/v{VERSION}.md, and the worker-mode instructions point at docs/installation/worker_mode.md.
   • The user_guide_src entries are removed from the four .gitignore files, and the root one is tidied (grouped sections, stale paths and the Vagrant entries removed).
   • README.md, tests/README.md and two @see docblock URLs in UpdateTest.php use the /develop/ Pages path, since the Pages root only redirects the index.
   • Dependabot has a weekly pip entry for docs/requirements.txt, grouped into one PR.

Notes

• The links assume the Pages layout from the switch PR, so merge this after it.
• Only the URLs change in the two test files.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide