Add an overview page for the TLS Handshake Modifier policy

Migrate or rewrite the TLS Handshake Modifier AI Gateway Policy page (`app/_ai_gateway_policies/tls-handshake-modifier/index.md`) to include the overview content from the corresponding plugin page: https://developer.konghq.com/plugins/tls-handshake-modifier/

Open app/_ai_gateway_policies/tls-handshake-modifier/index.md and compare it with the overview content on the corresponding TLS Handshake Modifier plugin page. Migrate or rewrite the policy page so it includes that overview content, then verify the resulting page covers the requested material.

Written by the indexing model from the issue text.