Fvwm now using Asciidoc for documentation

Hi all

I’ve just merged a change which switches the documentation away from docbook/XML to Asciidoc.

This therefore means you’ll need to install asciidoctor to build the man pages. The existing option of --enable-mandoc is still required.

I have removed HTML generation for now.

If you notice any problems, please open an issue on GitHub.

Thanks,
Thomas