[API] Add alert API docs

[nadouani]

Add documentation of the following APIs:

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

[whoisroot]

Please also document the 400 error response types too.