Split the section from main.rst into a separate file.  Reference it in the
index.rst.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20240109064628.595453-5-peterx@redhat.com


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