Add _git module to the API docs

[OpenBagTwo]

Is your feature request related to a problem? Please describe.

When I chose pygit2 as my backend (#16), I explicitly chose a design that:

Abstracts away all the calls to pygit2 into a module called _git so that if I decide later to switch to another library, I only have to do it in one place

That's essentially made the _git module into somewhat of a standalone library / module capable of general-purpose Git operations. As such, its docs should probably be included in the HTML APIdocs

Describe the solution you'd like

1. Audit the methods in the module and ensure that no "public" method either expects as input or returns a pygit2 object (.e.g init() should probably return None)
2. Update this line to not skip generating API docs for _git.py: https://github.com/OpenBagTwo/gsb/blob/35ec02734c7ad89f4fd9d97b89924d7276701830/docs/gen_apidocs.py#L12 or rename the module so that it doesn't start with an underscore

Describe alternatives you've considered

An alternative would be to spin off this module into its own stand-alone package.

Additional context

[OpenBagTwo]

Same goes for _make_zip_archive.py