Avoid using HTML in Markdown (#61104)
While the Markdown stdlib now supports HTML, it still cannot be turned
into valid LaTeX code, and hence will not appear correctly in the PDF
version of the manual. So best to avoid it.
I only left some uses of `<br>` in a Markdown table, as there is no good
way to replace them while keeping that table; see PR #61088 for my
attempt to address those.
See also issue #49991.
