[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.
So manpage compilation is not actually needed (except maybe for Windows, but that's another story). Maybe put in one of the commands that works under Linux for now and finish this PR. Manpages cannot current be installed anyway (conflicting file names with Vim manpages).