[Documentation] Unify API docs between Ember and Glimmer-VM

[pzuraq]

More and more APIs are being upstreamed into the Glimmer VM, including some of Ember's public APIs (which are then re-exported from Ember). This results in a duplication of docs, or docs being written in Ember when the APIs themselves are defined in Glimmer, which is not ideal. We should try to eventually figure out a path toward Ember consuming and re-publishing docs directly from the Glimmer VM, when the APIs are defined there, so we can have a single source of truth.