docs: block: Create blk-mq documentation

docs: block: Create blk-mq documentation

Create a documentation providing a background and explanation around the
operation of the Multi-Queue Block IO Queueing Mechanism (blk-mq).

The reference for writing this documentation was the source code and
"Linux Block IO: Introducing Multi-queue SSD Access on Multi-core
Systems", by Axboe et al.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20200620002036.113000-1-andrealmeid@collabora.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>