Refactor the API OAuth docs
What does this MR do?
Needed for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9095 as we weren't clear enough about how to retrieve the OAuth access token. I refactored the whole doc while at it. You can see the related changes in the two commits.
Author's checklist
Review checklist
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Technical writer's review (required)
Edited by Evan Read