[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.
Is there a way to install a newer version, e.g. via Homebrew?

Do you know how OS X handles manpages? As I wrote below, on Arch Linux, I think only the .1 files are installed by packages (e.g. /usr/share/man/man1/vim.1.gz), so this step should not even be necessary. I guess other Linux distros are similar, so maybe OS X is the only system where compiling manpages is actually necessary?