docs: Split features into new administration category (#1440)

* docs: Split features into new administration category

* docs: Add redirects for moved pages

docs/docs/administration/user-management.mdx

@@ -0,0 +1,24 @@
+import RegisterAdminUser from '../partials/_register-admin.md';
+import UserCreate from '../partials/_user-create.md';
+
+# User Management
+
+Immich supports multiple users, each with their own library.
+
+## Register the Admin User
+
+<RegisterAdminUser />
+
+## Create a New User
+
+<UserCreate />
+
+## Delete a User
+
+If you need to remove a user from Immich, head to "Administration", where users can be scheduled for deletion. The user account will immediately become disabled and their library and all associated data will be removed after 7 days.
+
+## Password Reset
+
+To reset a user's password, click the pencil icon to edit a user, then click "Reset Password". The user's password will be reset to "password" and they have to change it next time the sign in.
+
+![Reset Password](./img/user-management-update.png)