docs: publish documentation to GitLab Pages (MkDocs Material + CI)

- Add mkdocs.yml (Material theme, FR) building the user/dev guides from docs/
  into the site; internal app-store notes and image folders excluded.
- Add docs/index.md landing page; drop the now-redundant Home.md.
- Add .gitlab-ci.yml: a `pages` job (Linux, pinned mkdocs/mkdocs-material) that
  builds ./public for GitLab Pages on the default branch; plus a manual,
  macOS-tagged Swift test job.
- Fix FAQ links that pointed outside docs/ (→ repo URLs). Ignore build output.

Site build verified locally with the pinned versions.

Co-Authored-By: Claude <claude@anthropic.com>