Improve logical information in Sphinx indexes

[felixfontein]

I tried creating a docset for a collection for use in Dash or Zeal with doc2dash, and noticed that while the process works pretty well (except that the default theme isn't really suitable for this; according to https://gist.github.com/danieltomasz/d6aae37a0ba5f5ba0f17ea68b590731f the epub theme is better suited), with one big exception - the 'Sections' list:

The n copies of "Requirements" isn't exactly helpful (these are the requirements for every plugin and module), and the raw (internal) labels for plugins/modules/roles and their parameters and return values are even less helpful.

The latter can be improved by not adding labels to texts, but instead using a custom directive which generates the label and adds a nice title to it.

These things could also be added to an index (if you generate a PDF, you see that right now only the environmen variables show up in the index, and it is otherwise empty).