[AI Gateway] Clarify BYOK authorization headers

[Michael-WhiteCapData]

Summary

Clarifies that AI Gateway substitutes a stored provider key only when the provider authorization header is absent. Adds a DeepSeek BYOK request example that omits Authorization and uses cf-aig-authorization for the gateway.

Fixes #33517.

Documentation checklist

• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.