docs: Tweak location of qemu nbd extensions
Upstream QEMU has moved the location of its NBD docs, as of its commit
8dac93a8e[1]. Instead of pointing to a raw git .txt source file that
no longer exists, we now point to the rendered html version built from
rST [2].
[1] https://gitlab.com/qemu-project/qemu/-/commit/
8dac93a8e, see
also https://lists.gnu.org/archive/html/qemu-devel/2024-08/msg00223.html
[2] https://www.qemu.org/docs/master/interop/nbd.html
CC: qemu-devel@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>