docs: html cleanups
This MR fixes a handful of new warnings and errors from tidy sine last time I fixed things. Most changes are relatively trivial.
The three <pre>
-><code>
tags might be slightly opinionated, but the alternative is to insert lots of paragraphs to match the not very appealing rendering, which seems a bit crazy. So I hope the author here agree ;-)
Edited by Erik Faye-Lund