Generate man pages through Sphinx

Generate man pages through Sphinx

Instead of writing direct nroff markup from 'gmx help -export man',
generate the man pages using Sphinx from the same source that is used
for the HTML help.  This makes the code a lot more maintainable, and
enables using the same markup everywhere; where necessary for more
flexible formatting, the code can now be simply changed to emit direct
reStructuredText, and that get reasonably formatted in both contexts.
This should allow actually improving the man page contents in the
future.

For man pages, GMX_BUILD_HELP now only controls whether the installation
tries to install man pages or not; they are never built automatically,
as it now takes a substantial amount of time and requires Sphinx that
most people will not have available.

Currently the layout of the man pages (in particular gromacs.7) is not
very nice, but the situation is comparable to what it was earlier.

Change-Id: I3b581933369e573af5cbd614950c2a667a1c6fa4