Document how to delete accounts

The button works, but there is no process dealing with users set as retired, it still needs to be done manually.

Previous description (before 2021-07-29)

Logs:

lms_1            | 2020-12-09 12:31:06,561 INFO 14 [tracking] [user 2349] logger.py:49 - {"event": "{\"GET\": {}, \"POST\": {\"password\": \"********\"}}", "accept_language": "en-GB,en;q=0.5", "host": "learn.totem-project.org", "agent": "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:83.0) Gecko/20100101 Firefox/83.0", "event_type": "/api/user/v1/accounts/deactivate_logout/", "event_source": "server", "time": "2020-12-09T12:31:06.560906+00:00", "username": "maartentest", "referer": "https://learn.totem-project.org/account/settings", "page": null, "ip": "xxx.xxx.xxx.22", "context": {"path": "/api/user/v1/accounts/deactivate_logout/","course_id": "", "org_id": "", "user_id": 2349}}                                                                              
lms_1            | 2020-12-09 12:31:06,870 ERROR 14 [openedx.core.djangoapps.user_api.accounts.views] [user 2349] views.py:472 - 500 error deactivating account Default state does not exist! Populate retirement states to retire users.                     
lms_1            | Traceback (most recent call last):                                                                          
lms_1            |   File "/openedx/edx-platform/openedx/core/djangoapps/user_api/models.py", line 320, in create_retirement   
lms_1            |     pending = RetirementState.objects.all().order_by('state_execution_order')[0]                            
lms_1            |   File "/openedx/venv/lib/python3.5/site-packages/django/db/models/query.py", line 309, in __getitem__      
lms_1            |     return qs._result_cache[0]                                                                              
lms_1            | IndexError: list index out of range                                                                         
lms_1            |                                                                                                             
lms_1            | During handling of the above exception, another exception occurred:                                         
lms_1            |                                                                                                             
lms_1            | Traceback (most recent call last):                                                                          
lms_1            |   File "/openedx/edx-platform/openedx/core/djangoapps/user_api/accounts/views.py", line 423, in post        
lms_1            |     UserRetirementStatus.create_retirement(request.user)                                                    
lms_1            |   File "/openedx/edx-platform/openedx/core/djangoapps/user_api/models.py", line 322, in create_retirement   
lms_1            |     raise RetirementStateError('Default state does not exist! Populate retirement states to retire users.') 
lms_1            | openedx.core.djangoapps.user_api.models.RetirementStateError: Default state does not exist! Populate retirement states to retire users.