Add Google Gemini virtual integration docs (#38765)

source/_integrations/google_gemini.markdown

@@ -0,0 +1,21 @@
+---
+title: Google Gemini
+description: Instructions on how to integrate Google Gemini as a conversation agent
+ha_category:
+  - Voice
+ha_release: 2025.5
+ha_iot_class: Cloud Polling
+ha_config_flow: true
+ha_codeowners:
+  - '@tronikos'
+  - '@ivanlh'
+ha_domain: google_gemini
+ha_integration_type: virtual
+ha_platforms:
+  - conversation
+  - diagnostics
+ha_supporting_domain: google_generative_ai_conversation
+ha_supporting_integration: Google Generative AI
+---
+
+{% include integrations/supported_brand.md %}