Add an overview page for the Response Transformer Advanced policy

Migrate or rewrite the Response Transformer Advanced AI Gateway Policy page (`app/_ai_gateway_policies/response-transformer-advanced/index.md`) to include the overview content from the corresponding plugin page: https://developer.konghq.com/plugins/response-transformer-advanced/

Start with app/_ai_gateway_policies/response-transformer-advanced/index.md and compare it with the corresponding Response Transformer Advanced plugin page at https://developer.konghq.com/plugins/response-transformer-advanced/. Migrate or rewrite the policy page so it includes the plugin page's overview content; the work is done when the policy page presents that overview.

Written by the indexing model from the issue text.