Move away from injecting unstyled HTML into the sidebar, to using
Furo's default sidebar with Sphinx's `doctree` instead.
This also includes moving to a more typical Sphinx documentation
structure with the `index` page serving as the "root" of the `doctree`
for Sphinx.
Additionally, move custom stylesheets into a `pytest-custom.css` file
and use standard Sphinx tooling to inject these styles.
Pradyun Gedam