Improve accessibility of -T html -O toc output by using the <nav> element

Improve accessibility of -T html -O toc output by using the <nav> element
in the DPUB-ARIA doc-toc role.
Patch from Anna Vyalkova <cyber at sysrq dot in> slightly tweaked by me.

This is hopefully the start of a collaboration to improve accessibility
of Unix manual pages using the WAI-ARIA, HTML-ARIA, and DPUB-ARIA standards.
Progress appears to be possible without changing *anything* with respect to
the way manual pages are written.  Instead, it seems sufficient to properly
translate semantic cues already implied by existing mdoc(7) markup into the
appropriate HTML elements and ARIA attributes.  Overall, the total length
of HTML output is likely to increase slightly, but not much.