Allows users to start the sidebar Toc at level 0

Addresses issue pydata#1181

Copy function from toctree.py that needs to be modified (as-is for prettier diffs in future commits)

Rewrite toctree.resolve to an internal function that accepts `toc_caption_maxdepth` as a keyword argument

Use the new local _resolve function and pass it a new keyword argument: `toc_caption_maxdepth`.

Edit example pages and show commented out theme-parameters in conf.py

Change heading of sidebar-nav depending on `theme_navigation_startdepth`.
Shows "Site Navigation" when startdepth=0 or "Section Navigation" otherwise

…nd `toc_caption_maxdepth`.

Add gif to show intended behavior of collapsible toc captions.

Use `_traverse_or_findall` function instead of assuming `findall` is present.

Remove test for references to non-included documents. This feature was introduced sometime between Sphinx 5 and 6 apparently.

When `TocTree._toctree_prune` does not exist, use approach from Sphinx 6.1.3 for pruning the toctree.