Add basic documentation about NAPI. We can stop linking to the ancient
doc on the LF wiki.

Link: https://lore.kernel.org/all/20230315223044.471002-1-kuba@kernel.org/Reviewed-by: NBagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: NToke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Pavel Pisa <pisa@cmp.felk.cvut.cz> # for ctucanfd-driver.rst
Reviewed-by: NTony Nguyen <anthony.l.nguyen@intel.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Reviewed-by: NStephen Hemminger <stephen@networkplumber.org>
Reviewed-by: NRandy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20230322053848.198452-1-kuba@kernel.orgSigned-off-by: NJakub Kicinski <kuba@kernel.org>