docs/migration: Split "Debugging" and "Firmware"

Move the two sections into a separate file called "best-practices.rst".
Add the entry into index.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20240109064628.595453-6-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
3 files changed