docs nbd: userspace NBD now favors github over sourceforge

While the sourceforge site for userspace NBD still exists, the code
repository moved to github several years ago. Then with a recent
patch[1], the github landing page contains just as much information as
the sourceforge page, so we might as well point to a single location
that also provides the code.

[1] https://lists.debian.org/nbd/2023/03/msg00051.html

Signed-off-by: Eric Blake <[e-mail]>
Reviewed-by: Josef Bacik <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

Documentation/admin-guide/blockdev/nbd.rst

···
 Unlike NFS, it is possible to put any filesystem on it, etc.
 
 For more information, or to download the nbd-client and nbd-server
-tools, go to http://nbd.sf.net/.
+tools, go to https://github.com/NetworkBlockDevice/nbd.
 
 The nbd kernel module need only be installed on the client
 system, as the nbd-server is completely in userspace. In fact,