Improve logical information for labels to Ansible objects

[felixfontein]

Fixes #203.

Current version (Zeal 'Sections' screenshot of HTML docsite converted with doc2dash):

The n-fold 'Requirements' section still shows up, but at least the references to all return values and options look much better! Also, there's now an index with all modules, plugins, and roles (if you tell Sphinx to generate it):

[felixfontein]

More Zeal screenshots:

The requirements references look better:

Return values for function-like plugins (lookups, filters, tests):

Nested return values for function-like plugins (lookups, filters, tests):

Building the official docsite with an appropriate theme and converting it with doc2dash should now produce a pretty neat docset.

[felixfontein]

One problem is that the current solution works well with HTML generation, but not at all with LaTeX. I tried to play around with that the last days, but didn't had much time, and the only way I got it to work with both HTML and LaTeX writers resulted in a large amount of WARNINGS and CRITICAL errors (which did not impact functionality, since the collisions they complained about were intentional).

I'll try to spend some more time figuring out how to do this properly...

[felixfontein]

I think a better way is to allow doc2dash to change/remove labels with a (say) JSON file, and then dump such a JSON file from antsibull-docs. That allows us to obtain all these extra details without doing dirty tricks to try to make Sphinx output the titles for references that we want in the Interspinx file.