task: De-promote thirdweb — keep one tool page at parity with other tools, drop code examples and recommendations

Part of #2266.

### What needs to be done?

Decided: thirdweb stays in the docs as a listed tool but is **no longer promoted** — no more prominent than Hardhat, Foundry, viem or any other tool, no tutorials, no "recommended" wording.

At `bdf40b37` thirdweb appears in:

- 4 in-nav pages: `tooling/dev-environments/thirdweb/{overview,thirdweb,one-click-deploy}.mdx`, `tooling/libraries-sdks/thirdweb-sdk/index.mdx`
- 3 orphaned pages: `build-on-celo/build-with-thirdweb/{overview,celo-nft-drop-tutorial}.mdx` and `build-on-celo/build-with-thirdweb/one-click quickstart.mdx` (space in the filename; URL-encoded redirect source at `docs.json:3151`)
- a provider entry in `tooling/wallets/index.mdx:92`
- an "Alternative" section on `build-on-celo/build-with-ai/x402.mdx`
- 3 image assets (`img/thirdweb.svg`, `img/thirdweb-explore.png`, `img/building/how-to-deploy/thirdweb/thirdweb-explore.png`)
- a redirect chain at `docs.json:1468` → `/build/build-with-thirdweb/celo-nft-drop-tutorial`, which is itself a redirect source (line 3147)

### Definition of done

Code:
- [ ] The 7 pages collapsed into **one** neutral page: what it is, Celo chain configuration, link to thirdweb's own docs — same group and format as the other tools. OPEN: home it under dev-environments or under SDKs; pick one and apply the same choice to the redirects
- [ ] Tutorials, step-by-step code examples, and "recommended" / "easiest way" wording removed
- [ ] The 6 other paths redirect to the one page; `one-click quickstart.mdx` deleted
- [ ] Wallet-provider entry in `tooling/wallets/index.mdx` kept as a plain list item (parity with the other providers)
- [ ] x402 "Alternative" section reduced to one link
- [ ] Redirect chain at `docs.json:1468` resolved
- [ ] Unused images removed
- [ ] `npx mintlify broken-links` green

Ops (separate, outside the repo):
- [ ] Usage audit: which ecosystem projects still rely on thirdweb; result posted here. If usage is low, notify the top projects and plan the move to the cheaper alternative (≈$2k/year vs ≈$2k/month today)

### Non-goals

Removing thirdweb entirely; choosing or onboarding the replacement vendor.

### Depends on / blocks

Depends on #2252 (orphan-handling boundary — this ticket owns the 3 thirdweb orphans). Blocks #2259.

**Measured at:** `bdf40b37`