Integrate glossary across website

It would be great if pages across the website could refer to the glossary, so that specialised terms could be explained in popups.

Consult with @debruine who built a bookdown glossary plugin that might be very helpful here.

Start by reviewing how the website pages currently present specialised terms and consult @debruine about the bookdown glossary plugin. The work is complete when relevant terms across the website can be selected or hovered to show explanations in popups.

索引モデルが issue の本文から書いたものです。