Add GraphQL query cookbook (example queries by use case)

[dkijania]

Part of #1195 (Track 2).

Goal

A cookbook of practical, copy-pasteable GraphQL queries split by use case. Each example its own .mdx page so it's individually searchable, indexable, and linkable.

Why split into per-use-case pages

A single prose page (today's docs/node-developers/graphql-api.mdx) buries useful examples in long-form text. A flat list of focused pages — each titled by what the user is trying to do — is easier for both humans (search) and agents (URL-pattern navigation).

This complements the schema-generated reference (sibling issue): reference tells you what each field is; cookbook tells you which fields to combine for common tasks.

Proposed contents

Each as its own .mdx under docs/integrate/graphql-api/cookbook/:

• account-balance.mdx — Get an account's balance, nonce, delegate.
• watch-new-blocks.mdx — Subscribe to newBlock, filter by producer.
• estimate-fee.mdx — Use pooledUserCommands to estimate a sensible fee.
• find-transaction.mdx — Look up a transaction in the archive node.
• pending-transactions.mdx — Inspect the mempool.
• block-by-height.mdx — Fetch a specific block via archive GraphQL.
• delegation-info.mdx — Account staking and delegate chain.
• network-status.mdx — Sync state, peers, current epoch / slot.
• submit-payment.mdx — Mutation example (with --insecure-rest-server warning).

(Pick the ones that actually map to common questions; cut/add as needed.)

Page template

Each page should follow:

1. What this does (one sentence).
2. When to use it (one sentence — what real workflow needs this).
3. Query (fenced graphql block).
4. Variables (if any).
5. Example response (truncated).
6. Notes / gotchas (e.g. "memo is Base58Check encoded", "only last k blocks via daemon — use archive for older").
7. Related: links to relevant schema-generated reference pages and other cookbook entries.

Acceptance criteria

• Cookbook directory created with at least 6 starter recipes.
• Each page follows the template above.
• Sidebar entry under Integrate with Mina → GraphQL API → Cookbook.
• Cross-links between related recipes.
• Cross-links from each recipe to the relevant generated reference pages (depends on sibling issue).
• llms-full.txt regenerated.

Depends on

Section structure from #1195 Track 1. If that lands first, slot under docs/integrate/. Otherwise temporary path under docs/node-developers/graphql-cookbook/ and move later.