Update our HTML, CSS, and JS for Bootstrap 5

[choldgraf]

Context

The PyData theme updated its HTML and JS to support Bootstrap 5 instead of Bootstrap 4. We now need to upgrade our own HTML, CSS, and JS to support these changes.

This should happen after we merge:

• MAINT: Remove duplication with pydata-sphinx-theme #640

References

There's a guide to the changes in the PyData theme here that we can use for help.

The upgrade was made in:

• MAINT: Drop jQuery and use Bootstrap 5 pydata/pydata-sphinx-theme#1029