docs/migration: Create index page

docs/migration: Create index page

Create an index page for migration module.  Move VFIO migration there too.
A trivial touch-up on the title to use lower case there.

Since then we'll have "migration" as the top title, make the main doc file
renamed to "migration framework".

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/20240109064628.595453-3-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>