[RFC] Cleanup user documentation build

This PR does some cleanup in the Makefile of the user documentation.
The first commit makes `make html` work.
Also some generated files got deleted and the `*.1` files are converted to UTF-8.
@fwalch OS X uses a pretty old version of groff which doesn't support `-k`:
```
% groff -v
GNU groff version 1.19.2
Copyright (C) 2004 Free Software Foundation, Inc.
GNU groff comes with ABSOLUTELY NO WARRANTY.
You may redistribute copies of groff and its subprograms
under the terms of the GNU General Public License.
For more information about these matters, see the file named COPYING.

called subprograms:

GNU grops (groff) version 1.19.2
GNU troff (groff) version 1.19.2
```
It seems that this version doesn't support utf-8 as input encoding, though it supports utf8 as output encoding -.-