docs: migrate from Mintlify to Docusaurus 3

[planetf1]

What this does

This PR replaces the Mintlify documentation setup with a Docusaurus 3.10 site
that builds cleanly and deploys to GitHub Pages via a single CI job. The intent
is to give the team a live, navigable prototype at
https://planetf1.github.io/mellea/ while we verify the migration before
cutting over the canonical docs.mellea.ai domain.

The content of docs/docs/ is preserved almost entirely — changes are purely
mechanical: canonical frontmatter removed, sidebarTitle renamed to
sidebar_label, a handful of admonitions updated to Docusaurus syntax, and
internal links de-prefixed to match the way Docusaurus strips numeric filename
prefixes from doc IDs (01-your-first-generative-program → your-first-generative-program).

Key design decisions

Single docs plugin, two named sidebars. Rather than two separate
@docusaurus/plugin-content-docs instances, the site uses one plugin with
routeBasePath: '/' and both docsSidebar and apiSidebar defined in
sidebars.ts. This avoids SSR failures at the secondary plugin's root route
when only a placeholder API doc exists.

apiSidebar is autogenerated. The docs/docs/api/ tree is wiped and
regenerated by the autogen pipeline on every CI run. A static {type:'doc'}
entry in the sidebar would be deleted on each build; {type:'autogenerated', dirName:'api'} picks up whatever the pipeline emits. Locally, the placeholder
docs/docs/api/index.md acts as the seed.

SidebarFix injection disabled. decorate_api_mdx.py was emitting a
Mintlify-specific import { SidebarFix } from "/snippets/SidebarFix.mdx"
into every generated API MDX file. The snippet no longer exists on disk so
webpack fails. Step 3 in decorate() is now a no-op; no Mintlify artefacts
are needed in Docusaurus.

<Icon> shim added. mdxify generates <Icon icon="github" .../> as a
source link in every API heading. An Icon MDX component shim is registered
globally; it renders the GitHub SVG and handles the string-form style
attribute that mdxify produces.

onBrokenLinks: 'warn' — the generated api-reference.mdx and
reference/cli.md files contain cross-references that resolve incorrectly
against the Docusaurus route tree. These are a known deferred item (full
autogen pipeline retargeting). All static documentation pages are clean.

Fork/upstream base URL separation. DOCS_BASE_URL is set to /mellea/
when running on planetf1/mellea and / on generative-computing/mellea.
The same docusaurus.config.ts handles both cases via process.env.

What's included

• docs/docusaurus.config.ts — full config: single-plugin setup, 63 Mintlify
  redirect rules, IBM Plex Sans typography, local search, analytics script
• docs/sidebars.ts — 11-section manual sidebar plus autogenerated API sidebar
• docs/src/ — custom CSS (monochrome palette, sidebar badges), Mintlify shim
  components (Card, CardGroup, Icon, Note, Warning), global MDX registration
• docs/static/CNAME — docs.mellea.ai
• _category_.json for all 11 doc sections
• Autogen pipeline updated to guard against missing docs.json (Docusaurus mode)
  and to disable the Mintlify-only SidebarFix injection
• CI workflow updated: Node 20 setup, npm ci, Docusaurus build, deploy to
  gh-pages branch via peaceiris/actions-gh-pages

CI status

✅ Build and deploy passing on planetf1/mellea. Live at:
https://planetf1.github.io/mellea/

What's deferred

• Full autogen pipeline retargeting (sidebar TS emit, slug migration, MDX
  cross-reference cleanup — root cause of remaining onBrokenLinks: 'warn'
  entries)
• SPA analytics route-change module
• Soft-warning quality gates (9 new autogen checks)
• Algolia DocSearch, Cloudflare previews, snapshot versioning
• markdownlint-cli → markdownlint-cli2 bump

Testing

cd docs && npm ci && npm run build   # exits 0
npm run start                         # browse localhost:3000

Spot checks to verify:

• All 11 sidebar sections expand correctly
• Dark mode toggles
• Local search returns results
• A sample redirect (e.g. /guide/glossary → /reference/glossary) works
• API Reference navbar item shows the generated API tree