Detect badly formed document headers

[cgay]

To be included in the package docs an index.rst file should contain exactly one top-level header or else both headers will show up in the main Table of Contents. For example, the main ToC looks like this:

.. toctree::
   :hidden:
   :maxdepth: 1
   :caption: Packages

   anaphora <anaphora/index>
   base64 <base64/index>
   ...

and if anaphora/index.rst looks like this:

Anaphora
========

[...]
Anaphoric macros
----------------

This library currently implements:

* :macro:`aif`

Indices and tables
==================

* :ref:`genindex`
* :ref:`modindex`
* :ref:`search`

then both "Anaphora" and "Indices and tables" show up in the main ToC. We should be able to detect this with a scan of index.rst looking for any headers that are the same level as the first header.

[cgay]

Would be nice to give a warning about this when deft publish is being run, in addition to detecting it when gendoc runs, so the package author can fix it proactively.