Update Gloo Edge docs and examples to describe kgateway integration

[tjons]

Background
Solo.io donated Gloo to the CNCF, renaming it kgateway: https://kgateway.dev. The classic Gloo APIs have been replaced with the Kubernetes Gateway API. We've had docs on the site for using OPA Envoy Plugin with Gloo for years, but kgateway has proven very popular in the community due to vendor-neutral API support and AI gateway features. kgateway also supports Envoy's external authorization functionality, as well as external processing functionality: https://kgateway.dev/docs/security/external-auth/.

Let's review the current Gloo Edge docs and either replace or augment them with kgateway docs.

[7h3-3mp7y-m4n]

Hey @tjons, I saw that others have expressed interest already, but if they don’t follow up, I’d be happy to take this on! I’d really appreciate your help getting started when you have time. Thanks!

[charlieegan3]

We've just updated the OPA website, the Envoy docs we have can be found here: https://github.com/open-policy-agent/opa/tree/main/docs/docs/envoy. It'd be great to get this updated! Please lmk if you run into any issues with the new site dev process and feel free to @ me in any docs reviews.

[charlieegan3]

I can imagine we might want to review https://www.openpolicyagent.org/ecosystem/entry/gloo-api-gateway too? What would be the correct course of action for this page, just replace it with a kgateway page?