docs/migration: Create migration/ directory

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>

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