docs: kdoc_files: move output symbols logic to kdoc_output

docs: kdoc_files: move output symbols logic to kdoc_output

When writing unittests for kdoc_output, it became clear that
the logic with handles a series of KdocItem symbols from
a single file belons to kdoc_output, and not to kdoc_files.

Move the code to it.

While here, also ensure that self.config will be placed
together with set.out_style.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <4ebc26e37a0b544c50d50b8077760f147fa6a535.1773823995.git.mchehab+huawei@kernel.org>