Add table of content to each section + general improvements

[niccokunzmann]

Each file should have a table of content.

• This section navigation is empty however.
• You get section navigation with subdirectories and toctrees with captions. The PR I started for icalendar has that for Reference as an example.
  https://github.com/collective/icalendar/blob/83014bae7e7a6900a573f9dc1aef8de10168d650/docs/reference/icalendar.rst
• several pages that are short reference material, such as Media. You can group them into a section navigation item.