doc: make sure we use our own code for generating man pages
docutils would in some situations pick up its own manpage.py instead of
doc/manpage.py. Renaming to hgmanpage.py makes it less ambiguous.
*** failed to import extension badext from .../badext.py: bit bucket overflow
*** failed to import extension badext2: No module named badext2
hg help [TOPIC]
show help for a given topic or a help overview