story: MiniPay — slim the section to one overview that routes to docs.minipay.xyz

[GigaHierz]

Part of #2266.

User story

As a developer building a mini app, I want Celo's docs to tell me what is Celo-specific about MiniPay and send me to the MiniPay team's docs for the build/test/submit lifecycle — one current source instead of two drifting copies.

Context & evidence

At bdf40b37, build-on-celo/build-on-minipay/ has 5 pages — overview (1.9 KB), quickstart (9.5 KB), code-library (10.8 KB), deeplinks (1.8 KB), prerequisites/ngrok-setup (1.3 KB) — and none of them links to docs.minipay.xyz (grep -c docs.minipay.xyz → 0 on all five). The MiniPay team's own docs (https://docs.minipay.xyz — verified 200, with its own llms.txt) cover quickstart, testing inside MiniPay, wallet connection, deployment, submission and a technical reference (deeplinks, phone-number lookup). A second copy here is a second copy to keep fresh.

Decided: one strong overview; everything lifecycle-related routes out.

Acceptance criteria

• One page (build-on-minipay/overview.mdx or minipay.mdx) covering: why MiniPay (reach, distribution, discovery); what is Celo-specific (stablecoins, fee abstraction / paying gas in stablecoins, phone-number mapping, funding and program pointers); a clear "build, test, submit → docs.minipay.xyz" section with deep links to the matching MiniPay pages
• The other 4 pages deleted, with redirects to the overview (OPEN: external redirect to the matching docs.minipay.xyz page vs internal overview — default internal)
• Existing redirects at docs.json:1795–1816 re-pointed
• Celo-specific troubleshooting snippets kept per the code-example policy in AGENTS.md (task: Add AGENTS.md — repo guidance for structure, headings, writing style, and code-example policy #2251) — e.g. fee currency in a MiniPay transaction; the rest of code-library dropped
• Done in place — no directory move (story: Build tab — merge "Build on Celo" and Tooling into build/*, with Agents and Mini Apps leading #2259 moves it into the Mini Apps group)
• npx mintlify broken-links green

Ops (separate):

• MiniPay docs owners informed of the pages we deep-link to, and agreement that those slugs are stable

Non-goals

MiniPay growth/distribution content (after the restructure); the Build-tab move (#2259).

Depends on / blocks

Depends on #2251. Blocks #2259.

Measured at: bdf40b37