docs: sysfs-block: move to stable directory

The block layer sysfs ABI is widely used by userspace software and is
considered stable.

Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20211209003833.6396-2-ebiggers@kernel.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>