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 <[e-mail]>

Documentation/driver-api/index.rst

···
    mtdnand
    miscellaneous
    mei/index
+   mtd/index
    nvdimm/index
    w1
    rapidio/index

Documentation/mtd/index.rst Documentation/driver-api/mtd/index.rst

···
-:orphan:
-
 ==============================
 Memory Technology Device (MTD)
 ==============================

drivers/mtd/nand/raw/nand_ecc.c

···
  *   Thomas Gleixner ([e-mail])
  *
  * Information on how this algorithm works and how it was developed
- * can be found in Documentation/mtd/nand_ecc.rst
+ * can be found in Documentation/driver-api/mtd/nand_ecc.rst
  */
 
 #include <linux/types.h>