docs: md: move it to the driver-api book

The docs there were meant to be read by 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.
    firmware/index
    pinctl
    gpio/index
+   md/index
    misc_devices
    nfc/index
    dmaengine/index

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

-:orphan:
-
 ====
 RAID
 ====