docs: update user onboarding guide

docs/getting-started/user-onboarding.md

@@ -24,19 +24,13 @@
 
 === "For admin"
 
-    ## Create a new account
+    Run the following script:
 
-    TODO
+    ```sh
+    ./script/onboard-user exampleuser "Example User" "user@example.com"
+    ```
 
-    ## Send initial password
-
-    Choose one of the methods listed below to send the initial password to the user:
-
-    - Share via password manager (if supported)
-    - Encrypt the password file and send it via email or chat
-    - Simply write or print the password on a piece of paper
-
-    On the first login, the user will be required to update their password.
+    Let the user scan the QR code or follow the link to set up passkeys or password + TOTP.
 
 ## Appendix