Add an overview page for the Response Transformer policy

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

Start with app/_ai_gateway_policies/response-transformer/index.md and compare it with the overview content on the corresponding Response Transformer plugin page. Migrate or rewrite the policy page so it includes the relevant overview content, then verify that the page renders correctly and covers the plugin overview.

Written by the indexing model from the issue text.