Add GitHub Pages documentation site

[theworker02]

Summary

• Wire MkDocs Material on the existing docs/site/ tree with ShiftLock brand colors, logo, and grouped nav
• Add .github/workflows/pages.yml to build and deploy via official Pages actions (push to main + workflow_dispatch)
• Flesh home / intro / quick-start, keep site deps in requirements-docs.txt (out of go.mod), and document the expected Pages URL in the README

Test plan

• Confirm mkdocs build --strict -f docs/site/mkdocs.yml succeeds (verified locally)
• In repo Settings → Pages, set source to GitHub Actions if not already
• After merge (or via workflow_dispatch on main), confirm deploy job succeeds
• Visit https://theworker02.github.io/shiftlock/ and check home, quick start, search, and theme
• Confirm CI still passes and go.mod has no docs/Python deps

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