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.
I'd probably opt to change the reference in the target to point to this specifically, rather than add cruft to this file.