docs: kernel_include.py: generate warnings for broken refs

docs: kernel_include.py: generate warnings for broken refs

In the past, Sphinx used to warn about broken references. That's
basically the rationale for adding media uAPI files: to get
warnings about missed symbols.

This is not true anymore. So, we need to explicitly check them
after doctree-resolved event.

While here, move setup() to the end, to make it closer to
what we do on other extensions.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/73be9a198746421687e2eee916ccf8bf67980b7d.1755872208.git.mchehab+huawei@kernel.org