docs: mmc: move it to the driver-api

Most of the stuff here is related to the kAPI.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/driver-api/index.rst

@@ -45,6 +45,7 @@ available subsections can be seen below.
    miscellaneous
    mei/index
    mtd/index
+   mmc/index
    nvdimm/index
    w1
    rapidio/index

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

-:orphan:
-
 ========================
 MMC/SD/SDIO card support
 ========================