[Docs] Document Gemini 3.7 Flash support

[zoomote]

​Opened on behalf of @taltas. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• Documented Gemini 3.7 Flash for the direct Google Gemini and GCP Vertex AI providers.
• Added the model ID, context and output limits, reasoning controls, multimodal support, global Vertex endpoint requirement, and introductory pricing period.
• Updated the Gemini model-catalog link to Google's current documentation.

Why this change was made

The provider guides need to match Zoo Code's new Gemini 3.7 Flash support and Google's generally available release details.

Impact

Users can configure Gemini 3.7 Flash through either supported Google provider with clear endpoint, capability, and pricing guidance.

Related PRs

• Zoo Code model support

Screenshots