doc: misc-device: add uacce to toctree(index)

This fixes:

Documentation/misc-devices/uacce.rst: WARNING: document isn't included in any toctree

Signed-off-by: Vitor Massaru Iha <vitor@massaru.org>
Link: https://lore.kernel.org/r/20200430225828.114033-1-vitor@massaru.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>