Expose include-all-objects as global Sphinx option?

For Gammapy I would like to adopt the default of `include-all-objects` globally for the docs.

How about adding a `automodapi_include_all_object` global Sphinx option?

I think it's pretty common to want everything I list in `__all__` (e.g. module-level variables like default values or dictionaries) to show up in the docs, no?

For now, is there a way to achieve this default globally for Gammapy?
Or do I have to add `:include-all-objects:` to every `automodapi` directive?

Start by reading the automodapi directive and the Sphinx configuration options involved in processing it. Determine how a global include-all-objects setting could apply to every automodapi directive, and verify that the resulting documentation includes objects listed in __all__ without requiring the directive option each time.

Written by the indexing model from the issue text.