ci: deploy mkdocs via GitHub Actions

[lesnik512]

Summary

• Adds a Deploy Docs workflow that auto-deploys mkdocs to gh-pages on push to main when docs/** or mkdocs.yml changes.
• Adds a docs-deploy recipe to Justfile (uvx mkdocs gh-deploy --force), mirroring modern-di.
• Configures the custom subdomain semvertag.modern-python.org via site_url in mkdocs.yml and docs/CNAME.
• Bumps action pins across ci.yml, publish.yml, semvertag.yml, tag-major.yml to match modern-di parity: checkout@v6, setup-just@v4, setup-uv@v8.2.0.

Spec: planning/specs/2026-06-09-mkdocs-github-actions-design.md

Test plan

• CI lint job passes under bumped action pins
• CI pytest matrix passes under bumped action pins
• action-smoke job passes under checkout@v6 (this is the one job where the bump could surface a difference — composite action exercised via uses: ./)
• After merge: trigger Deploy Docs via workflow_dispatch from the Actions UI — workflow succeeds, creates gh-pages branch
• After merge: enable GitHub Pages on the gh-pages branch in repo Settings → Pages
• After merge: configure DNS for semvertag.modern-python.org (CNAME → modern-python.github.io.) and verify the site serves

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