Add support for :noindex:

[bsipocz]

Sphinx 2.1 became more strict and now complains about duplicated object descriptions:

Warning, treated as error:
duplicate object description of astropy.nddata.utils, other instance in nddata/index, use :noindex: for one of them
Sphinx Documentation subprocess failed with return code 2
Exited with code 2

However currently the automodapi directive doesn't support noindex (WARNING: Found additional options noindex in automodapi.)
and no-heading doesn't do the trick.

[adrn]

Just re-discovered this! One workaround is to use the :no-main-docstr: option, but then you lose the main docstring (not a problem in my use cases).