docs: migrate the documentation site into the monorepo

[najuna-brian]

What

Moves the Docusaurus site from the standalone OpenDataEnsemble/docs repository into this monorepo at docs/, so documentation changes ship in the same PR as the code they describe.

The docs repository history is preserved: the site was imported with git subtree add --prefix=docs, so all 140 commits from OpenDataEnsemble/docs are ancestors of this branch. git log HEAD~3^2 -- docusaurus.config.ts walks the original docs history.

Layout

docs/                     Docusaurus site root (independent npm project)
docs/docs/                content root: getting-started, using, guides, reference, community, ...
docs/static/              assets + CNAME + .nojekyll
docs/src/                 custom components, theme overrides, /downloads page
.github/workflows/docs.yml

No URL changes

url, baseUrl: '/' and routeBasePath: '/docs' are unchanged, so https://opendataensemble.org/docs/... keeps working. The site stays an independent npm project (the rest of the monorepo uses pnpm); docs/package-lock.json is committed via a .gitignore exception.

Fixes made along the way

• editUrl pointed at a non-existent ode-docs/ directory; it now resolves to ode/tree/dev/docs/docs/<page>.
• projectName / organizationName now identify OpenDataEnsemble/ode.
• The repo-root docs/custom-question-types.md becomes a published page, docs/docs/guides/custom-question-types.md, linked from Custom Extensions.
• Contributor docs updated: repo URLs, dev as the default branch, community forum links, and the site README's incorrect versioning notes (versioning is disabled, versions.json is empty).
• Dropped files the site never used: a nested workflow, create-placeholders.sh, an unreferenced sidebars-api.ts, and a stale copy of the Synkronus OpenAPI document.
• CNAME moved into static/, where Docusaurus actually publishes it.

CI

.github/workflows/docs.yml runs npm ci, npm run test and npm run build (with onBrokenLinks: 'throw') on pull requests touching docs/**, and deploys to GitHub Pages on pushes to dev. Locally verified: 104 pages build, build/CNAME is opendataensemble.org, .nojekyll present.

⚠️ Cutover required after merge

OpenDataEnsemble/docs currently owns the opendataensemble.org custom domain, and only one Pages site can hold a domain. The deploy will fail until that domain is moved to this repository. Plan:

1. Disable the old repo's Documentation CI & Deploy workflow.
2. Merge this PR; the first deploy publishes to the github.io fallback URL, proving the pipeline.
3. Release opendataensemble.org from OpenDataEnsemble/docs and assign it to OpenDataEnsemble/ode.

[r0ssing]

Looks great! I suggest we update to deploy from 'main' (I added code suggestions where needed). If you agree, please feel free to accept those and merge the PR 🦖

Then we can update the repo settings together afterwards....