[API] Add alert API docs

[nadouani]

Add documentation of the following APIs:

• [x] Create alert
• [x] Update alert
• [x] List/Search alert
• [x] Mark alert as read/unread
• [x] Merge alert into a case
• [x] Promote alert into a case
• [x] Delete an alert
• [x] List alert similar cases
• [x] List alert observables
• [ ] Add alert observable
• [ ] Update alert observable
• [x] Delete alert observable
• [x] Run responder on alert
• [ ] List responder jobs
• [ ] Review

[whoisroot]

Please also document the 400 error response types too.