Add an overview page for the XML Threat Protection policy

Migrate or rewrite the XML Threat Protection AI Gateway Policy page (`app/_ai_gateway_policies/xml-threat-protection/index.md`) to include the overview content from the corresponding plugin page: https://developer.konghq.com/plugins/xml-threat-protection/

Start with app/_ai_gateway_policies/xml-threat-protection/index.md and compare it with the overview content on the corresponding XML Threat Protection plugin page. Migrate or rewrite that overview content in the AI Gateway Policy page, then verify the page contains the intended overview information.

Written by the indexing model from the issue text.