Add an overview page for the Route Transformer Advanced policy

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

Start with app/_ai_gateway_policies/route-transformer-advanced/index.md and compare it with the corresponding Route Transformer Advanced plugin page at https://developer.konghq.com/plugins/route-transformer-advanced/. Migrate or rewrite the page so it includes the plugin page’s overview content; done means the AI Gateway Policy page contains that overview.

Written by the indexing model from the issue text.