Migration documentation is growing into a single file too large. Create a
sub-directory for it for a split.
We also already have separate vfio/virtio documentations, move it all over
into the directory.
Note that the virtio one is still not yet converted to rST. That is a job
for later.
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20240109064628.595453-2-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
8cb2f8b172