[WIP] 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.
`nroff` somehow gets the encoding wrong for utf8 files. Does somebody have an idea how to fix this?
Maybe we don't need this at all as manpages get shipped as .1 files.