How to use captions in navigation sidebar when using autosummary?

[Connossor]

Hi,

Firstly, thanks for an absolutely fantastic library! Sphinx is proving tremendously useful.

I was wondering if it's possible to include a Caption when using autosummary. I'm using the ReadTheDocs theme. Currently my index.rst looks a bit like this:

Group1

======

.. autosummary::

   :toctree: _autosummary

   package1
   package2

Group2

======

.. autosummary::

   :toctree: _autosummary

   package3

   package4

I wish to separate Group1 and Group2 in the navigation sidebar. I believe it's the "caption" TOC option. For example, as shown in this example: "THEME DOCUMENTATION" and "DEMO DOCUMENTS" are nicely separated. 

But this doesn't seem to be accepted when using autosummary. The code below fails. Any ideas?

Group1
======


.. autosummary::
   :toctree: _autosummary
   :caption: Group 1

   package1
   package2



Group2
======


.. autosummary::
   :toctree: _autosummary
   :caption: Group 2

   package3
   package4

Any advice greatly appreciated!