Unable to build documentation using default Sphinx selection

[braxtonj]

When running sh build_docs.sh, am finding the the following error (MacOS Ventura):

Theme error:
An error happened in rendering the page develop.
Reason: UndefinedError("'style' is undefined")

Digging a bit, seems to be due to incompatibility with Sphinx 7.0 as found in this kivy issue

To fix, modified the docs/requirements.txt to specify sphinx==6.2.1 as they suggest

[braxtonj]

Created PR 60