Add ocis idm resetpassword subcommand

This allows resetting the ocis admin user's password (when ocis is
not running) when running with the builtin LDAP server.

Fixes #4084

docs/services/idm/admin_password_reset.md

@@ -0,0 +1,24 @@
+---
+title: Resetting a lost administrator password
+date: 2022-08-29:00:00+00:00
+weight: 10
+geekdocRepo: https://github.com/owncloud/ocis
+geekdocEditPath: edit/master/docs/services/idm
+geekdocFilePath: admin_password_reset.md
+geekdocCollapseSection: true
+---
+
+## Resetting a lost administrator password
+By default, when using oCIS with the builtin IDM an ad generates the
+user `admin` (DN `uid=admin,ou=users,o=libregraph-idm`) if, for any
+reason, the password of that user is lost, it can be reset using
+the `resetpassword` sub-command:
+
+```
+ocis idm resetpassword
+```
+
+It will prompt for a new password and set the password of that user
+accordingly. Note: As this command is accessing the idm database directly
+will only work while ocis is not running and nothing else is accessing
+database.