docs(protocol): rewrite the protocol overview for the L2 and drop the stale outdated-docs warning

[GigaHierz]

The hole, and the fix

home/protocol/index.mdx was a 225-word stub that opened with a warning telling readers the docs "may be outdated" — a permanent negative signal on one of the highest-intent protocol pages, 17 months after the migration it warned about. Its body still described the retired L1 ("based on go-ethereum") and linked two legacy/* pages plus L1 consensus.

Rewritten as a ~740-word overview of the chain that exists today: OP Stack architecture with the two Celo substitutions (EigenDA data availability, token-duality precompile), settlement and the challenger/Security Council roles, fees (below a cent, gas in stablecoins via CIP-64), Mento stablecoins, staking/epochs after the migration (validators → community RPC operators), governance, identity (SocialConnect/ODIS, Self, ERC-8004), and what the protocol is optimized for (mobile users, AI agents). Warning banner deleted.

Per AGENTS.md "one fact, one page": every number is a link to its canonical page (/specs/deployments, /specs/transaction-fees, /specs/eigenda, /specs/token-duality, fee-abstraction guide) rather than a restated value.

What this does NOT do / residual risk

• Does not touch home/protocol/celo-token.mdx's similar stale warning — Deduplicate migrated Specs pages against the Home and Build guides #2227 owns that page.
• Links only to pages that exist on main today; two links get better targets after open PRs merge (phone-number mapping detail → docs(minipay): collapse the section to one Celo-specific overview that routes to docs.minipay.xyz #2271's section; a dedicated Self Agent ID page → docs(agents): add Self Agent ID page under Agent Identity and turn the Self page into a journey #2270) — both work either way.
• Word count ~740, slightly under the "~800" in story: Rewrite the Celo Protocol overview — remove the stale L1 warning banner, describe the current L2 #2282 — density over padding; happy to extend if wanted.

Judgement calls

• Framed identity through MiniPay/SocialConnect and Self rather than the deleted legacy ODIS pages, consistent with docs: remove the Legacy tab #2280's direction.
• Did not state the 80/20 fee burn/carbon split: /specs/transaction-fees still marks the final L2 behaviour unconfirmed, so the page links instead of asserting.

Issues

Closes #2282 — all boxes met (banner gone, current-chain overview, canonical links, no legacy links, broken-links green).

Stacking / conflicts

Branched off main. #2280 edits the same file (2-line legacy-link fix); this rewrite supersedes those lines — whichever lands second rebases, resolution is "take the rewrite".

Verification evidence

$ mint broken-links   # on 3ddb86c0
success no broken links found
$ git show :home/protocol/index.mdx | grep -c "may be outdated"
0

Facts asserted on the page were verified earlier in this workstream: 1-second blocks and fee level pinned to the specs pages; maxNumGroupsVotedFor/epoch mechanics per the #2280 review (on-chain); EigenDA/settlement per /specs/eigenda and cel2-architecture.

Remaining ops steps

• none

Checklist

• Title is the commit message I want on main
• mint broken-links green; no legacy or redirect-source links
• Judgement calls flagged above
• No secrets in the diff