docs: rename the Home tab to Learn and move home/ to learn/

[GigaHierz]

Closes #2258

The "Home" tab is now Learn. home/* moves to learn/*, the two protocol-reference pages that sat in the builder tab move in with it, and every old URL redirects to its new home.

What moved

• 42 pages: home/<path> to learn/<path>, same sub-paths (for example home/protocol/governance/overview is now learn/protocol/governance/overview).
• Wrong-tab fix: build-on-celo/network-overview to learn/network/overview, and build-on-celo/cel2-architecture to learn/network/architecture.
• Nav groups in the Learn tab: Introduction, Network, Protocol, Governance, Transactions, Staking, Epoch Rewards, Bridging, and a new "Using CELO" group (wallets, exchanges, ramps, gas fees, self-custody, ReleaseGold, asset management). Every page is listed exactly once; the two moved pages are removed from the Build tab.
• .github/CODEOWNERS: /home/ and /home/protocol/ become /learn/ and /learn/protocol/, so the blockchain and primitives teams keep ownership.
• AGENTS.md: content directories and the canonical-page table now use the new paths.

Redirects (all in docs.json)

• Added /home to /learn and the catch-all /home/:slug* to /learn/:slug*, so home/celo and every other old page redirect without a per-file override (the paths are 1:1 below learn/).
• Added exact entries for the two moved build-on-celo pages.
• Retargeted every existing redirect that pointed at /home/... or at the two moved pages straight to the new paths, so there are no two-hop chains. This includes the wildcard ones (/about-celo/:slug*, /what-is-celo/using-celo/*).
• Removed the old /learn/:slug* to /home/:slug* wildcard. With real /learn/... pages it would shadow them and loop. The explicit /learn/<old-slug> redirects stay, retargeted.
• Updated 50 files of inbound links (every /home/... link and the two moved pages).

Verification

$ mint broken-links --check-redirects
success no broken links found
$ bash scripts/check-orphans.sh
No orphan pages found.

Under mint dev, 15 old URLs all redirect in one step to the new path, for example /home/protocol/celo-token, /home/manage/exchange, /build-on-celo/network-overview, /about-celo/protocol, /learn/why-celo, and every new learn/... URL returns 200. The tab bar reads Learn, Build on Celo, Tooling, Contribute to Celo, Operate.

Not in this PR

• Page titles and first paragraphs are not rewritten (the epic says story: Learn tab — move home/* (plus kept L1 content) to learn/* #2258 to story: Operate tab — move infra-partners/* to operate/* and fold Specs in as a Specification section #2260 move paths and do not rewrite content).
• The end-user MetaMask and Ledger setup pages stay under Tooling for now: docs(metamask): point end users at the add-network button and the wallets page #2355 changes the same files, and the MetaMask setup page is a developer page, so which of them move needs a decision first.
• home/wallets, home/exchanges and home/ramps moved rather than being deleted; task: Make sure every end-user project listed in the docs is also on celo.org/ecosystem #2265's ecosystem decision is untouched.
• mint dev shows /learn going on to /learn/celo; on production /home serves its index page directly. Worth a look after deploy.

Blocks #2261.

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