Build fixes

Fixes (almost) all the HTML build errors on the manual. There remains the issue of the missing logo file, but that's not really a problem.

Of note: for whatever reason, in `source/includes/steps-backup-sharded-clusters-dumps.yaml` , if there are line breaks in the warning text, it doesn't build properly, so I've made it all one big long line. Not very pretty, but it generates properly.
Done!