docs: write API authentication guide

[basanth-p]

Summary

Write the API authentication guide for developers integrating the Patch ID score API.

Content to cover

1. API key generation — how to get a key from the partner console
2. Request authentication — Authorization: Bearer <key> header
3. Rate limits — tiers and limits per plan
4. Error codes — 401, 429, 403 with explanations
5. Example requests — cURL, JavaScript fetch, Python requests

Deliverable

/api-reference/authentication.mdx (or equivalent in chosen docs framework)