sphinx site name defaults to package guide instead of pyOpenSci - it should be configurable in each book that uses this theme

[lwasser]

https://github.com/pyOpenSci/pyos-sphinx-theme/blob/9bd1cad48b8a37133843e88e428f8d2714a31bd8/src/pyos_sphinx_theme/__init__.py

in the init.py file, we config the site name to default to pyopensci python package guide. however this theme is used across all of our books so we should make that configurable via sphinx vs hard coded here.