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/mtd/index.rst

@@ -0,0 +1,10 @@
+==============================
+Memory Technology Device (MTD)
+==============================
+
+.. toctree::
+   :maxdepth: 1
+
+   intel-spi
+   nand_ecc
+   spi-nor