Add live GraphiQL playground page pointed at devnet

[dkijania]

Part of #1195 (Track 2).

Goal

Embed a live, in-browser GraphQL explorer in docs2, pointed at a public Mina devnet endpoint. Read-only. Lets developers (and agents that drive a real browser) try queries against real chain data without spinning up a node or copying queries elsewhere.

Why live, not mocked

A live devnet endpoint is more useful than a mock and avoids the maintenance cost of keeping mocks aligned with the schema. The tradeoff is the playground breaking when devnet is down — acceptable for a dev tool. If devnet rate-limits or ToS become a problem, add a thin Cloudflare Worker proxy with caching as a follow-up.

Approach

1. New page docs/integrate/graphql-api/playground.mdx.
2. Use @graphiql/react (or graphql-playground-react if smaller bundle) inside an MDX page. Both are React, both work in Docusaurus.
3. Default endpoint: a public devnet GraphQL URL (decide which one — there are a few; pick one operations is happy to keep up).
4. Read-only: hide or disable the mutation tab, or proxy through a worker that rejects mutations. Simpler: client-side filter on the operation kind before the request fires.
5. Pre-populate the query editor with a useful starter query (e.g. bestChain(maxLength: 1) { stateHash creator }).
6. Pin a small CSS fix so the GraphiQL component plays nicely with Docusaurus's layout (full-height container).

Acceptance criteria

• Page renders and connects to the chosen devnet endpoint.
• Query / docs / schema-explorer panes work.
• Mutations are blocked or hidden.
• Works on the Vercel preview deployment.
• Sidebar entry added under Integrate with Mina → GraphQL API.
• Bundle size impact noted in PR description (GraphiQL pulls in a Monaco-ish editor — flag it).

Depends on

Endpoint decision — needs ops sign-off on which devnet host is OK to advertise as "the playground endpoint." Tag the relevant team in the PR.