Use ``sphinx_rtd_theme`` instead of our custom one.
Start restructuring the docs to have a nicer navtree.
Reorder / -structure more to have nice TOCs again.
Fix broken inheritance digram creation (seems it wasn't working for quite a while)
Remove unreferenced section index documents.  Fix most sphinx errors / warnings.
Merge branch 'feature/rtd_theme_339'

* feature/rtd_theme_339:
  Remove unreferenced section index documents.  Fix most sphinx errors / warnings.
  Fix broken inheritance digram creation (seems it wasn't working for quite a while)
  Reorder / -structure more to have nice TOCs again.
  Start restructuring the docs to have a nicer navtree.
  Use ``sphinx_rtd_theme`` instead of our custom one.