This PR creates the sphinx extension package plasmapy_sphinx. This package defines two reST directives automodapi and automodsumm, which are an adaption of the likely named directives from sphinx_automdoapi. There are a few significant distinctions of our directives over the sphinx_automodapi directives that made me add the code here and not push it upstream to sphinx_automodapi.

1. I introduced the concept of groups for the directives. So, when an module is inspected for its objects, the found objects are categorized into group types (modules, functions, classes, exceptions, etc.). As a result, the sphinx_automodapi options like :functions-only: are replaced by :groups: functions.

2. This move to the groups options allows for multiple groups to be displayed at once for automodsumm by doing

   .. automodsumm:: foo.bar
      :groups: functions, classes
   
   .. automodapi:: foo.bar
      :groups: functions, classes

   It additionally allows for custom groups to be defined.

3. Configuration values (that are defined in conf.py) are introduced that allows you define custom groups. To define a custom group the automodapi_custom_groups configuration value needs to be defined like

   automodapi_custom_groups = {
       "mygroup" : {
           "title": "My Custum Group",
           "dunder": "__mygroup__",
       },
   }

   In this example mygroup would be the name of the custom group, "title" would be the title rendered above the autosummary table when automodapi is used, and "dunder" is the name of the dunder variable used to list the custom group objects. This __mygroup__ would be placed in the module being documented with the custom group and is defined just like __all__.

4. The automodapi directive is an official autodoc directive and not a function connected to the source-read event. This allows automodapi to benefit from all the default functionality associated with the autodoc directives.