Fix up/down keyboard navigation for 'Show documentation' links

The navigation widget relies on a crude HTML "parser" which
counts "lines". Yes. Only '<br/>' is considered a newline, even
though due to line wrapping we can have obviously more lines. Well,
without rewriting all of that, also count '</p>' as introducing a
new line. This allows us to jump to the 'Show documentation' link
then.

To make this work reliably, we also fixup the generated HTML to be
valid and not nest '<p>' tags. Furthermore, we prevent the ugly
line indication marker '<->' being shown when we try to down past
the last paragraph.