Instead of the custom <div class="manual-text">, use the standard

Instead of the custom <div class="manual-text">, use the standard
HTML <main> element.  The benefit is that it has the ARIA landmark
role "main" by default.  To ease the transition for people using
their own CSS file instead of mandoc.css, retain the custom class
for now.

I had this idea in a discussion with Anna Vyalkova <cyber at sysrq dot in>.
Patch from Anna, slightly tweaked by me.