Improved formatting for sidebar navigation

[qualiaMachine]

Now that we have three high-level categories (Learn, Toolbox, and Applications), the sidebar is looking a little cluttered.

As one possible improvement, we could use bold text for the high-level headers (Home, Learn, Toolbox, and Applications, etc.). I'm not sure if that's a straightforward adjustment using quarto. I think we might (?) have some luck with other themes (rather than "cosmo"), but haven't done any digging on that yet.

Alternatively, we could collapse the high-level headers as a default view. However, collapse-level=1 appears to be buggy. I ran into this behavior: quarto-dev/quarto-cli#27. Until that bug is fixed, I don't think this route is a viable option.

@UW-Madison-DataScience/nexus-developers this is up for grabs if anyone has time to do some sleuthing.