✨ ENH: Make starting depth of sidebar configurable in generated html

[jorisvandenbossche]

This generalizes how we currently select (with soup.select("li.current.toctree-l1") the "active" subset of the sphinx-generated html for the sidebar, that allows both disabling it (eg for a single sidebar) as setting it at a higher value (eg for a deeper nested page, where the navbar already selects two levels, eg with dropdown).

It adds a startdepth keyword for this, following the maxdepth terminology of sphinx (but we could certainly also go for something more explicit / readable if there are suggestions)

[choldgraf]

This looks good to me - thanks for all the careful testing as well! One thing I note is that there aren't any major docs changes. However, it seems to me that this is mostly an "under the hood" change that makes it easier to use the theme in a "single sidebar" or a "topbar" mode. So there don't need to be any user-facing docs changes but perhaps we could add something to the developer docs? I don't think we need to block the PR on that, but WDYT @jorisvandenbossche ?

[jorisvandenbossche]

Yes, this should certainly be documented. I would maybe first like to experiment a bit further with using it / creating a single sidebar version, to ensure it's fully working as I want, before documenting it (although this might sound like a good excuse to then eventually not do it ... ;-))