Added information about prompt caching for Gemini models (#56754)

Author: justb0b

content/copilot/reference/ai-models/how-copilot-serves-ai-models.md

@@ -49,4 +49,6 @@ Used for:
 
 {% data variables.product.prodname_copilot %} uses {% data variables.copilot.copilot_gemini_flash %} and {% data variables.copilot.copilot_gemini_25_pro %} hosted on Google Cloud Platform (GCP). When using {% data variables.copilot.copilot_gemini %} models, prompts and metadata are sent to GCP, which makes the [following data commitment](https://cloud.google.com/vertex-ai/generative-ai/docs/data-governance): _{% data variables.copilot.copilot_gemini %} doesn't use your prompts, or its responses, as data to train its models._
 
+To provide better service quality and reduce latency, {% data variables.product.github %} uses [prompt caching](https://cloud.google.com/vertex-ai/generative-ai/docs/data-governance#customer_data_retention_and_achieving_zero_data_retention).
+
 When using {% data variables.copilot.copilot_gemini %} models, input prompts and output completions continue to run through {% data variables.product.prodname_copilot %}'s content filters for public code matching, when applied, along with those for harmful or offensive content.