Add a quickstart tutorial & re-organize index page

We split docs into tutorials & more in-depth guides.

docs/guides/admin.rst

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+Administrative Access
+---------------------
+
+In The Littlest JupyterHub, we try to allow users to do as many administrative
+tasks as possible within JupyterHub itself. Admin users can:
+
+1. Have full root access with passwordless ``sudo``
+2. Install systemwide packages with ``apt``
+3. Install ``conda`` / ``pip`` packages for all JupyterHub
+4. Change amount of RAM / CPU available to each user and more!
+
+By default, there are no admin users. You should add some after installation.
+
+Adding admin users
+==================
+
+Admin users are specified in the `YAML <https://en.wikipedia.org/wiki/YAML>`_
+config file at ``/opt/tljh/config.yaml``.
+
+1. Open the ``config.yaml`` file for editing.
+
+   .. code-block:: bash
+
+      sudo nano /opt/tljh/config.yaml
+
+2. Add usernames that should have admin access.
+
+   .. code-block:: yaml
+
+      users:
+        admin:
+          - user1
+          - user2
+
+   Be careful around the syntax - indentation matters, and you should be using
+   spaces and not tabs.
+
+   When you are done, save the file and exit. In ``nano``, you can do this with
+   ``Ctrl+X`` key.
+
+3. When you are sure the format is ok, restart JupyterHub to let the config take
+   effect.
+
+   .. code-block:: bash
+
+      sudo systemctl restart jupyterhub
+
+This should give you admin access from JupyterHub! You can verify this by:
+
+1. Opening a Terminal in your JupyterHub and checking if ``sudo`` works
+2. Opening your JupyterHub ``Control Panel`` and checking for the **Admin** tab
+
+From now on, you can use the JupyterHub to do most configuration changes.