I don't remember why I introduced these when switching to pandoc, but
they are not absolutely needed. Instead of pre-generating them (which
might save some trivial cpu time), we can simply pass footer.rst as an
additional input file to pandoc.

The advantage in doing so is that semantic information is preserved;
when pre-generating and including them as-is, the entire structure is
lost, as pandoc won't reinterpret them, so things like a potential
table of contents will simply ignore the footer.

The patch also adds a new target to rebuild all man pages (both groff
and html formats), for testing changes to the pages easier.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>