Add an overview page for the Proxy Caching Advanced policy

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

Open app/_ai_gateway_policies/proxy-cache-advanced/index.md and compare it with the corresponding Proxy Caching Advanced plugin page at the linked URL. Migrate or rewrite the page so it includes the plugin page’s overview content; the issue is done when the policy page presents that overview.

Written by the indexing model from the issue text.