docs: update API rate limits

[samrathreddy]

Summary

• update the public rate-limits page to match the current backend request-rate defaults
• distinguish request-rate 429 responses from credit/spend-cap 402 responses
• update synthetic data, training job, and API key endpoint pages so the inline limit notes match the consolidated table

Validation

• git diff --check
• stale-value search for old documented limits
• mint dev --no-open served the updated rate-limits page at http://localhost:3001/api-reference/rate-limits
• mint validate runs under Node 22, but fails on existing site config issues: favicon generation treats the remote DuckDuckGo favicon as a local path, and strict validation fails on an existing OpenAPI warning from docs.json.