story: Learn tab — move home/* (plus kept L1 content) to learn/*

Part of #2266.

### User story

As someone asking "what is Celo", or holding CELO, I want one tab with protocol concepts, governance, history and end-user guidance — instead of a tab called "Home" that is really protocol reference.

### Context & evidence

At `bdf40b37` the "Home" tab (`docs.json:38`) has 30 pages in groups Introduction / Overview / Protocol / Governance / Transactions / Epoch Rewards / Manage applications. `build-on-celo/network-overview.mdx` and `build-on-celo/cel2-architecture.mdx` are protocol reference sitting in the builder tab.

### Scope

- Rename tab **Home → Learn**; move `home/*` → `learn/*` with the group structure from the #2209 Appendix A map at `a14395f8926eb43424e0d20e6d67ab96f2c8209b`: Introduction / Network architecture / Protocol / Governance / Stablecoins & bridging concepts / Epoch rewards / Identity / Using CELO
- Pull `build-on-celo/{network-overview,cel2-architecture}` in (wrong-tab fix)
- Include whatever #2254 classified as "keep → Learn"
- Include the end-user MetaMask / Ledger setup pages as split in #2256
- Redirects: catch-all `{ "source": "/home/:slug*", "destination": "/learn/:slug*" }` plus per-file overrides for every non-1:1 move
- Titles state the audience per `AGENTS.md` (#2251)

### Acceptance criteria

- [ ] Tab renamed; every Learn page reachable from nav
- [ ] Redirects in place: catch-all + overrides; verified by curling 10 old URLs → 200 at the new path
- [ ] Inbound links from other tabs updated
- [ ] `home/celo.mdx` (the landing page) keeps its path or gets a redirect — #2261 decides its final home; do not rewrite it here
- [ ] `npx mintlify broken-links` + orphan check green

### Non-goals

Homepage copy (#2261); content rewrites beyond titles and first paragraphs. CODEOWNERS: `home/` is owned by the blockchain and primitives teams — request their review.

### Depends on / blocks

Depends on #2251, #2252, #2253, #2254, #2256. Do not delete `home/wallets`, `home/exchanges`, `home/ramps` before #2265 lands. Blocks #2261.

**Measured at:** `bdf40b37`