Update the documentation referencing Plan 9 from User Space.

The page originally referenced to checkout Plan9 application and libraries
have been missing for quite some time and the development is carried out
in github and documented on this new site.

Signed-off-by: Juan Manuel Méndez Rey <vejeta@gmail.com>
Link: https://lore.kernel.org/r/20200426015250.GA35090@camelot


Signed-off-by: Jonathan Corbet <corbet@lwn.net>