docs/rest: Add a top level docs page for REST endpoints

[coco98]

Let's add a top level page as well or move the existing docs page to the top level page since it's already a good guide!

https://hasura.io/docs/latest/graphql/core/api-reference/restified.html

Some suggested additions:

1. Add a bit more focus on feature usage. Add examples/sample use cases
2. Section on creating rest endpoins with Console/CLI/API workflows
3. GraphQL / REST error codes