GCP integration: add connect-providers page

* Add an example walkthrough in https://aigateway.envoyproxy.io/docs/getting-started/connect-providers/
* Edit "supported providers" page to reflect the change: https://aigateway.envoyproxy.io/docs/capabilities/llm-integrations/supported-providers

<img width="1298" height="225" alt="Image" src="https://github.com/user-attachments/assets/75b3ed7c-9d75-419e-b742-7b6f7d36f886" />
