Add hover tips for glossary cross-references

[niksirbi]

This depends on #69, and should be tried after we have a minimal glossary published.

The idea it to have a tooltip appear on hovering over a term, so you can see its definition at a glance without the need to open a new page. It's not an essential feature, but more of a nice-to-have.

Sphinx Tippy is probably what we need to do this.