docs/migration: Split "Postcopy"

Split postcopy into a separate file.  Introduce a head page "features.rst"
to keep all the features on top of migration framework.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20240109064628.595453-7-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
4 files changed