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

While I was tempted to move it to admin-guide, as some docs
there are more userspace-faced, there are some very technical
discussions about memory error correction code from the Kernel
implementer's PoV. So, let's place it inside the driver-api
book.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/driver-api/index.rst

@@ -44,6 +44,7 @@ available subsections can be seen below.
    mtdnand
    miscellaneous
    mei/index
+   mtd/index
    nvdimm/index
    w1
    rapidio/index

Documentation/driver-api/mtd/index.rst

@@ -1,5 +1,3 @@
-:orphan:
-
 ==============================
 Memory Technology Device (MTD)
 ==============================