docs: move "Using Kuadrant" portion in user guides into a README of user-guides directory

[KevFan]

Description

Resulting from a converstaion to decouple Kuadrant AuthPolicy from the authorino user guides, it's suggested that we move the "Using Kuadrant" section from the user guides to a README.md under authorino/docs/user-guides in the github repo since Kuadrant Operator AuthPolicy is a consumer of the AuthConfig .

This would link users to Kuadrant if they're considering Gateway API while also separating out that section from the authorino user guides