Add the documentation site foundation (T11)

[wallstop]

The repository has no documentation site, so #114's documentation work has no foundation. This adds the site skeleton, two pages, one strict local build command, and a docs-only Pages workflow.

Behavior

• mkdocs.yml derives the site URL from the repository owner and name, keeps a two-page nav with light and dark palettes, and fails the build on an unlisted page or an unresolvable link or image. scripts/build-docs.ps1 builds it strictly from a pinned requirements-docs.txt, and the workflow calls the same script.
• The workflow builds on pull requests and on main, then deploys on every main push that touches documentation. Pages is configured with the GitHub Actions source and has no deployment yet, so the first merge publishes the site. Two pages only, and no imagery until chore: Documentation OverHaul + Auto-Captured Images/webp #114's capture render gap is fixed, so the README does not link to the site yet.

Validation

• Strict build green (3 pages); red with exit 1 on a missing page link, a missing image, and an unlisted page. -Serve previews at the configured site path. actionlint 1.7.7 clean.
• npm run lint:llm:full green (18/18 self-test files); npm pack payload unchanged at 172 files; the release .unitypackage still builds and validates with 92 entries. No C# or Unity change, so the Unity suites are not exercised.

Risk / Rollback

• Risk: low. Docs, one build script, one workflow; no package, editor, or runtime behavior change. The first main run attempts the repository's first Pages deployment.
• Rollback: revert these commits. The workflow is path-filtered to documentation files.

Refs #114, T11. The pre-existing CSharpier gate failure found while validating is #129.