Add listing of programs by topic to HTML output.

Now the HTML-exported help contains also a list of programs by topic,
similar to what used to be generated from programs.txt.  Removed the
mkhtml script, since it is now fully replaced by the mechanism in the
wrapper binary.

The same mechanism could also be used to replace the gromacs.7 man page,
but in its current form, it contains so much boilerplate code that I
didn't want to copy-paste that all into the source file.  And it could
also be used to make the output of 'gmx help' more structured.

Related to #685, #969, and #1242.

Change-Id: I6c2efe10c53f10f7fde90b3386ddea7fbea34b89