Add docs for API keyset pagination

What does this MR do?

This adds documentation for the REST API keyset pagination method.

It is currently only supported for the projects endpoint when ordering by id.

The actual feature hasn't been merged yet (or better: it was reverted). I extracted the docs to a separate MR to discuss this individually. Once approved, I'll merged it as soon as the actual feature lands.

The same goes for a changelog entry - I'll add that with the feature code.