Make the docs stroy line more clear to simplify navigation

[fwitte]

Idea

• Landing page
• User guide
  • Installation
  • Tutorials on the modeling of very basic systems (explanatory with storytelling)
  • Tutorials on more advanced systems (explanatory with storytelling)
  • More examples on advanced methods
  • Example applications of oemof-solph, that are very suffisticated (links to external libraries)
• Documentation
  • Meta documentation on the different parts of oemof (walk through kind of style as in the current user's guide)
    • general overview
    • components
    • investment optimization
    • ...
  • API docs
  • Changelog
  • Literature (?)
• Contribute
  • What to contribute
  • How to contribute
  • oemof association information

The high level points should have neat overviews to illustrate the contents of the different subchapters. Blueprint version of this is the tespy toc