docs(ai-gateway): clarify OTEL authorization config (#31171)

The authorization field is now optional in the API (previously required
customers to pass an empty string). Update docs to:
- Clearly mark which fields are required vs optional
- Explain that authorization is a secret store reference, not a raw token
- Document the headers.Authorization alternative for passing auth directly
- Add content_type field to the docs

Related: https://gitlab.cfdata.org/cloudflare/aig/aig-worker-config-api/-/merge_requests/624