AI Gateway: Document AI Sanitizer plugin external PII service interface

## Info

The AI Sanitizer plugin docs mention that users can swap the default PII detection image for their own, but do not document the contract a custom service must implement to work with the plugin.

Add an interface reference covering:

- Endpoints the plugin calls (paths, methods)
- Request payload format the plugin sends
- Expected response payload format (including how sanitized values are returned)
- Authentication expectations
- Whether credential sanitization uses a separate contract from PII sanitization

## DoD

- Reference section added to the AI Sanitizer plugin docs documenting the full plugin-to-service contract
- Users can build and validate a custom PII service implementation from the docs alone
- Existing mention of swappable images links to the new reference

## Size

S

Start with the AI Sanitizer plugin documentation and locate the existing mention of swappable PII detection images. Document the plugin's endpoints, methods, request and response payloads, authentication, and credential-sanitization distinction, then link that reference from the existing mention. Done when a user can build and validate a custom PII service from the docs alone.

Written by the indexing model from the issue text.