[API] Add case API docs

[nadouani]

Add documentation of the following APIs:

• [ ] Create case
• [ ] Update case
• [ ] List/Search cases
• [x] Merge cases
• [x] Delete case
• [x] Export case to MISP
• [x] List related cases
• [x] List related alerts
• [x] List attachments
• [x] Run responder on case
• [x] List responder jobs
• [ ] Review