docs: nfc: add it to the driver-api book

Most of the descriptions here are oriented to a Kernel developer.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/driver-api/index.rst

@@ -56,6 +56,7 @@ available subsections can be seen below.
    pinctl
    gpio/index
    misc_devices
+   nfc/index
    dmaengine/index
    slimbus
    soundwire/index

Documentation/nfc/index.rst → Documentation/driver-api/nfc/index.rst

-:orphan:
-
 ========================
 Near Field Communication
 ========================