Add an overview page for the Redirect policy

Migrate or rewrite the Redirect AI Gateway Policy page (`app/_ai_gateway_policies/redirect/index.md`) to include the overview content from the corresponding plugin page: https://developer.konghq.com/plugins/redirect/

Start with app/_ai_gateway_policies/redirect/index.md and compare it with the overview content on the corresponding Redirect plugin page. Confirm the policy page includes the relevant overview material, then verify the rendered documentation page for completeness and formatting.

Written by the indexing model from the issue text.