Add an overview page for the Exit Transformer policy

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

Start with app/_ai_gateway_policies/exit-transformer/index.md and compare it with the overview content at https://developer.konghq.com/plugins/exit-transformer/. Migrate or rewrite the policy page so it includes the corresponding plugin overview content, then verify that the page is complete and consistent with the source.

Written by the indexing model from the issue text.