docs: fix links to point to freedesktop.org

docs: fix links to point to freedesktop.org

kmscon is now hosted on freedesktop.org. Please don't use the github
links, anymore. I will push to github for some more weeks, but I recommend
to everyone to use my freedesktop.org repository as primary upstream link.

There were many reasons why github.com wasn't sufficient, anymore. But the
main reason is that they no longer provide upload space for release
tarballs. Hence, lets ditch github and move to freedesktop.org which
kindly provides everything we need plus a really nice mailing-list.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>