Document the dedicated pydantic-ai vendor connection types

[Lee-W]

Azure OpenAI, Bedrock, and Vertex AI each have their own connection type and hook in this provider, with fields the generic pydanticai connection does not offer, but none of them appeared anywhere in the docs. The generic connection page meanwhile still taught Bedrock and Vertex AI setups that predate them, and its Vertex example relied on a credential fallback the underlying provider does not implement.

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