Node+VuePress TLC

[rgwood]

A few website updates:

• Upgrade GitHub Actions to Node 16 (from 12). 16 is the latest LTS release and 12 will be unsupported soon.
• Upgrade VuePress minor version
• Install the VuePress back-to-top and medium-zoom plugins; they were previously causing build warnings because they were enabled but not installed.
• Exclude the old Nu book from search results; it was making the search results a lot worse (so many duplicates!)

Testing Performed

Manual testing: clicked around the website, confirmed that the search works as expected in both English and German, confirmed that the VuePress plugins now work.

Details

The back-to-top and medium-zoom plugins were set up in config.js but not installed; previously the build was generating these warnings:

warning An error was encountered in plugin "@vuepress/plugin-back-to-top"
warning An error was encountered in plugin "@vuepress/plugin-medium-zoom"

Adding the --debug flag to vuepress dev or vuepress build was enough to confirm that it was unable to find those plugins entirely. I'm not a VuePress expert by any means, but I looked around GitHub and confirmed that they need to be installed in package.json.

[sophiajt]

thanks!