docs: kdoc; Add a rudimentary class to represent output items

docs: kdoc; Add a rudimentary class to represent output items

This class is intended to replace the unstructured dict used to accumulate
an entry to pass to an output module.  For now, it remains unstructured,
but it works well enough that the output classes don't notice the
difference.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>