Better support & reference docs for Gateway API

*Description*:
We currently have this doc https://gateway.envoyproxy.io/v0.5.0/user/gatewayapi-support.html which outlines the fields within Gateway API that EG supports, doesnt support, and/or can be customized in a EG specific way.

The end user currently needs to reference the upstream Gateway API reference https://gateway-api.sigs.k8s.io/references/spec/ and then peek into the above EG Gateway API support doc to figure out if the API field will actually work or not.

Raising this issue to consider adding upstream Gateway API Spec docs + specific EG comments, instructions to it
within this project, to simplify user experience