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>