setup: define build_doc command Currently, various processes for packaging Mercurial state to manually invoke `make -C doc` in order to generate the documentation. This Makefile merely invokes `gendoc.py` and `runrst` to produce man pages and HTML pages. Not all environments may have the ability to easily run Makefiles. Windows is notably in this set. This commit ports the man page and HTML generation logic from doc/Makefile to setup.py. We introduce a new build_doc command which generates documentation by calling gendoc.py and runrst. The documentation can now be built via pure Python by running `python setup.py build_doc`. We don't implement dependency tracking because IMO it is more effort than it is worth. We could potentially remove the duplicated functionality in doc/Makefile. But I'm not sure what all is depending on it. So I plan to keep it around. # no-check-commit because forced foo_bar function names Differential Revision: https://phab.mercurial-scm.org/D6063