Add an overview page for the GraphQL Proxy Caching Advanced policy

Migrate or rewrite the GraphQL Proxy Caching Advanced AI Gateway Policy page (`app/_ai_gateway_policies/graphql-proxy-cache-advanced/index.md`) to include the overview content from the corresponding plugin page: https://developer.konghq.com/plugins/graphql-proxy-cache-advanced/

Start with app/_ai_gateway_policies/graphql-proxy-cache-advanced/index.md and compare it with the overview content on the corresponding GraphQL Proxy Caching Advanced plugin page. Migrate or rewrite the policy page so it includes that overview content, then verify the rendered page is complete and consistent with the plugin documentation.

Written by the indexing model from the issue text.