story: Homepage, Contribute rename, and an "AI resources" page — final pass

Part of #2266.

### User story

As a first-time visitor (human or agent), I want the root of docs.celo.org to route me by what I'm trying to do, and to find in one place everything the docs offer to AI tools.

### Context & evidence

At `bdf40b37`: the first page of the first tab is `home/celo.mdx` (title "Celo"), which already leads with agent cards (ERC-8004, x402, MPP, Celopedia) — 8 mentions of "agent". `home/index.mdx` ("What is Celo?") has 0. "Contribute to Celo" (`docs.json:363`) has 17 pages under `contribute-to-celo/`. What already ships for AI tools and is documented nowhere in one place: `llms.txt` (200, 39 KB), `llms-full.txt` (200, 1.5 MB), per-page `.md` export (200), the contextual menu (copy / view / mcp / chatgpt / claude / cursor / vscode), the hosted MCP endpoint at `docs.celo.org/mcp` (405 on GET — exists), the Celo MCP server page, Celopedia, the installable skills.

### Scope — three small pieces, one final pass

1. **Homepage.** Make `home/celo.mdx` the real root landing page with jobs-to-be-done cards — build an agent that pays / build a Mini App / accept stablecoin payments / what is Celo / run a node — linking into the four tabs. Drop unexplained jargon from it ("Token Duality", "native transfer"). `home/index.mdx` becomes Learn's overview.
2. **Contribute.** Rename "Contribute to Celo" → "Contribute"; `contribute-to-celo/*` → `contribute/*`; catch-all redirect.
3. **AI resources page** under Build > Agents listing everything above, with one line on how to use each; add the assistant from #2250 once it is live.

### Acceptance criteria

- [ ] Root URL serves the card homepage
- [ ] Contribute renamed, paths shortened, redirects verified
- [ ] AI-resources page live, linked from the homepage and from `use-docs-with-ai.mdx`
- [ ] Final full-site `npx mintlify broken-links` + orphan check green
- [ ] Epic status table closed out

### Non-goals

New agent content (#2263). The Startup Pathway link is **not** re-added (decided; it went with the Celo Camp removal in #2235).

### Depends on / blocks

Depends on #2258, #2259, #2260. #2250 only for the assistant line — not a blocker.

**Measured at:** `bdf40b37`