Updated the docs on how to operate nomad. Now there is a documentation of the...

Updated the docs on how to operate nomad. Now there is a documentation of the CLI (few use cases and command reference) and the whole doctree has been modified for better navigation. Also added a new dev-dependency for sphinx-click that can autogenerate docs for click.

docs/cli.rst

+Command Line Interface (CLI)
+----------------------------
+The :code:`nomad` python package comes with a command line interface (CLI) that
+can be accessed after installation by simply running the :code:`nomad` command
+in your terminal. The CLI provides a hiearchy of commands by using the `click
+package <https://click.palletsprojects.com/>`_.
+
+This documentation describes how the CLI can be used to manage a NOMAD
+installation. For commmon use cases see :ref:`cli_use_cases`. For a full
+reference of the CLI commands see :ref:`cli_ref`. 
+
+.. toctree::
+   :maxdepth: 2
+
+   cli_use_cases.rst
+   cli_ref.rst