support generating multi-html pages in parallel

Use the pattern from other projects where we generate the html pages
in a dir named the same as the project.  So now we have:
gas/doc/gas.html - single html page
gas/doc/gas/ - multiple html pages

This works for projects that have a doc/ subdir already, but gprof &
ld require a little tweaking since they generate their docs in their
respective toplevels.