<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=468013) </details> <!--IssueSummary end--> MR: Pending ## Description As a follow up to https://gitlab.com/gitlab-org/gitlab/-/issues/433324+, add documentation that explains why the Web IDE uses OAuth and how. For context, the OAuth application is created at the code level and can be found [here](https://gitlab.com/gitlab-org/gitlab/-/blob/482f1e629ba2585f12f7434f3a4359a59b1f66d1/lib/gitlab/web_ide/default_oauth_application.rb#L46-49).