Documentation: Add io_ordering.rst to driver-api manual

Add io_ordering.rst under Documentation/driver-api and reference it from
the Sphinx TOC Tree present in Documentation/driver-api/index.rst

Signed-off-by: Pragat Pandya <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/driver-api/index.rst

···
    isa
    isapnp
    io-mapping
+   io_ordering
    generic-counter
    lightnvm-pblk
    memory-devices/index