"Ubuntu Mono" is not rendered correctly (used in "pre" CSS)

"Ubuntu Mono" is not rendered correctly, see adambard/learnxinyminutes-docs#5408 for screenshots; I don't know what causes it, but it is present on all (3) Ubuntu installations I checked, in all browsers (both Firefox and Chrome based ones) - I recommend removing it or replacing with "DejaVu Sans Mono", which doesn't have this problem.

Also for rendering ASCII diagrams like https://learnxinyminutes.com/docker I recommend adding `line-height: 1` for "pre".