Replace gromacs.7 man page with gmx.1

Replace gromacs.7 man page with gmx.1

The description is the same, and the command list is there, but other
content is omitted.  Useful parts can be added back later, when it is
clearer what we want to include here.  Also, the man page now contains a
synopsis and common options for the gmx binary, as is suitable for
gmx.1.

Part of #1437.

Change-Id: I4dbdf542dd2b8a0a7a2e734eebdf13d68483842e