Re-add user env files

docs/howto/user-env/notebook-interfaces.md

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+(howto/user-env/notebook-interfaces)=
+
+# Change default User Interface
+
+By default, logging into TLJH puts you in the classic Jupyter Notebook
+interface we all know and love. However, there are at least two other
+popular notebook interfaces you can use:
+
+1.  [JupyterLab](http://jupyterlab.readthedocs.io/en/stable/)
+2.  [nteract](https://nteract.io/)
+
+Both these interfaces are also shipped with TLJH by default. You can try
+them temporarily, or set them to be the default interface whenever you
+login.
+
+## Trying an alternate interface temporarily
+
+When you log in & start your server, by default the URL in your browser
+will be something like `/user/<username>/tree`. The `/tree` is what
+tells the notebook server to give you the classic notebook interface.
+
+-   **For the JupyterLab interface**: change `/tree` to `/lab`.
+-   **For the nteract interface**: change `/tree` to `/nteract`
+
+You can play around with them and see what fits your use cases best.
+
+## Changing the default user interface
+
+You can change the default interface users get when they log in by
+modifying `config.yaml` as an admin user.
+
+1.  To launch **JupyterLab** when users log in, run the following in an
+    admin console:
+
+    ```bash
+    sudo tljh-config set user_environment.default_app jupyterlab
+    ```
+
+2.  Alternatively, to launch **nteract** when users log in, run the
+    following in the admin console:
+
+    ```bash
+    sudo tljh-config set user_environment.default_app nteract
+    ```
+
+3.  Apply the changes by restarting JupyterHub. This should not disrupt
+    current users.
+
+    ```bash
+    sudo tljh-config reload hub
+    ```
+
+    If this causes problems, check the [logs](#troubleshoot-logs-jupyterhub) for
+    clues on what went wrong.
+
+Users might have to restart their servers from control panel to get the
+new interface.