Generate gromacs.7 from the wrapper binary

Use the same mechanism for generating gromacs.7 as with the by-topic
list of programs for the HTML pages.  Now the man page content is
up-to-date, no longer requires programs.txt, and uses the new
gmx-something names for the referenced programs.

Part of #685

Change-Id: I070f88fd366b37d1ba287a3c2e8e26a846774002