Add EE-specific GraphQL schema documentation

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29998 adds GraphQL schema documentation. It currently requires a manual run (see https://gitlab.com/gitlab-org/gitlab-ce/issues/64397 to improve that) of a Rake task, but there's a trickier problem: it only contains the Core fields. Any fields or objects or whatever in other editions aren't covered.

Initially, we could solve this by running the Rake task in the EE repo, and then committing the resulting documentation to the CE repo. Once we have a single codebase this would be the default anyway.

We should also be able to highlight which parts are in which edition, though, and that is not easy. @.luke pointed out how we can figure out EE-specific fields on resources in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29998#note_191003838, but that still doesn't tell us which edition (Starter, Premium, Ultimate) something is in.