feat(docs): seed Docusaurus site at /site, deploy to docs.fallout.build (#41)

[ChrisonSimtian]

Summary

Phase 1 of #41 — Documentation site migration.

• Stands up Docusaurus 3.10 in /site/, reading content from the existing ../docs/ tree (single source of truth — no fork).
• Targets https://docs.fallout.build via Cloudflare Pages (GitHub Pages slot is held by the apex fallout.build).
• Adds .github/workflows/docs-deploy.yml — builds and deploys on push to main when site/**, docs/**, or the workflow itself changes.
• Touch-ups to 8 docs files for strict MDX 3 compile (HTML → MDX comments, drop two redundant {#anchor} IDs).

Full architectural notes — including why same-repo + /site folder, why Cloudflare Pages, and how the deploy step is the v13 CD-POC reference for ADR-0001 — are in the commit body (d697b15).

Manual setup required before the deploy job will succeed

The build half works on any push. The deploy half needs (one-time):

1. Create Cloudflare Pages project named fallout-docs (Cloudflare dashboard).
2. Repo secrets: CLOUDFLARE_API_TOKEN (Pages:Edit), CLOUDFLARE_ACCOUNT_ID.
3. DNS on fallout.build: CNAME docs → fallout-docs.pages.dev.
4. Cloudflare Pages project → Custom domains → add docs.fallout.build.

If we merge before these are wired up, the workflow will run on push to main and fail at the deploy step. Two options:

• Wire Cloudflare setup first, then merge — deploy succeeds on the merge commit.
• Merge now, accept one red docs-deploy run, then wire Cloudflare and re-run.

Explicit non-goals (follow-up PRs)

• Custom favicon/logo (Docusaurus default placeholder retained).
• Algolia DocSearch wiring (requires OSS-plan application).
• Cleaning up broken-link warnings from numbered-prefix paths (onBrokenLinks: 'warn' keeps CI green).
• Per-PR preview deploys.

Test plan

• CI green on ubuntu-latest (the only PR-gating runner).
• cd site && npm ci && npm run build succeeds locally with the included package-lock.json.
• After Cloudflare setup, post-merge docs-deploy run is green and https://docs.fallout.build serves the site.
• Homepage at / renders; /docs/introduction resolves; navbar GitHub link works.

Closes part of #41 (Phase 1). Done-when checklist on #41 stays open — search, link audit, README link are follow-up.

[ChrisonSimtian]

Closing — this approach was superseded after the branch was last touched.

The docs site is live at https://docs.fallout.build/, but via a different architecture:

• Built and deployed from a sibling repo: ChrisonSimtian/Falloutdocs — holds Docusaurus config, theme, components, and the deploy workflow.
• Markdown source-of-truth stays here in docs/. Falloutdocs' deploy workflow checks out both repos and reads markdown from ./fallout-source/docs.
• Hosted on GitHub Pages (not Cloudflare Pages). The Fallout repo's Pages slot is taken by the apex fallout.build, so the sibling repo owns the docs subdomain — one custom domain per repo limit.
• Triggers: push to Falloutdocs main, daily 02:00 UTC schedule (picks up Fallout docs/ changes within 24h), manual workflow_dispatch.

Nothing in this PR belongs on main:

• /site/ scaffold → lives in Falloutdocs instead.
• .github/workflows/docs-deploy.yml → replaced by Falloutdocs' deploy workflow.
• The 8-file MDX 3 touch-ups → already merged separately as chore(docs): MDX 3 compat (HTML→JSX comments, drop {#anchor} in introduction) (#41 follow-up) #182.

Branch will be deleted. Follow-ups on #41 tracked in new issues.