docs(site): add hand-authored Astro docs site

Adds a hand-authored Astro 6 documentation site under site/, following the postern pattern (shared chrome from the public-sector reference design CDN, no Starlight). It deploys to this repo's GitLab Pages at https://gitlab-com.gitlab.io/public-sector/manifold/.

What is here

• Layout and libs: hand-authored Layout.astro (reference CDN CSS, Phosphor icons, Inter and JetBrains fonts, theme toggle, clipboard, heading permalinks), nav.ts, and a withBase() URL helper. Top-nav actions link the live demo (manifold-demo) and the source repo.
• 13 pages: home; Start (getting started, deploy to a customer instance); Guides (large instances, airgapped deployment); Reference (CLI and container commands, configuration, data schema, how scoring works, security model); Contributing.
• Content: migrated from the five docs/*.md files (now removed) and the README deploy section. README trimmed to a lean landing that links the docs site and the live demo. Non-markdown assets kept.
• CI: build-docs (merge requests touching site/, and the default branch) plus pages (default branch only). Purely additive; the Go build, sign, test, and release jobs are unchanged.

Shelves manifold's documentation: docs site, live demo, and source are now mutually linked.