Refactor: doc: Get rid of sphinx templates.

The custom class and module templates were present because the
autogenerated summaries for those things were not at all detailed -
basically, no members of classes and modules would be output. This in
turn led to warnings when sphinx would try to generate docs for
inherited members, especially in the IntEnum class.

But, we don't need those templates at all if we also don't use
autosummary. This module is supposed to do a bunch of documentation
generation for us, but it causes various problems - the above, and it
doesn't seem to work with the singlehtml builder that we default to.

So, get rid of the templates and future commits will shift away from
using autosummary.

Fixes T726