Documentation: mono: Update links and s/CVS/Git/

The URLs in mono.rst redirect to pages on www.mono-project.com, so let's
update them. I took the liberty to update the compilation instructions
to the Linux-specific version, because readers of the kernel
documentation will most likely use Linux.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>