Update Docsy

Updates PrismJS and Docsy.

Unfortunately Docsy wants to go more and more into the npm ecosystem (and I really don't), so we have to include Font Awesome in our repo from now on. I added some lines to the README to quickly describe how to update the fonts.

Marked as Draft because there is currently a bug in Docsy that makes the search icon not rendering (see https://github.com/google/docsy/issues/1256), I assume the release will be retagged with this fix included. Let's wait until then...