Add an overview page for the Route By Header policy

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

Start with app/_ai_gateway_policies/route-by-header/index.md and compare it with the overview content on the corresponding Route By Header plugin page. Migrate or rewrite the page so it includes that overview content, then verify the resulting documentation is complete and accurate.

Written by the indexing model from the issue text.