Revise the documentation

[uvchik]

There are still some old parts in the documentation but also new parts. We need to bring everything in line and decide what to keep and what to remove.

• [x] Changelog vs. Whatsnew: I merged both variants in changelog.rst. Now we have to decide.
  • [ ] Shall we use the old structure or one file?
  • [ ] Shall we use changelog or whatsnew (I would prefer changelog)
• [ ] Check all important parts (Installation, examples, usage)
• [ ] Compile sphinx without warnings (or at least few warnings)
  • [x] First sketch without warnings and broken links
• [x] Fix all references
• [ ] Missing content?
  • [ ] Basic usage.
  • [ ] General information: Overview weather data/models
• [ ] ...

See this branch on readthedocs

[uvchik]

@oemof/feedinlib : I would appreciate some help, ideas and feedback.

[birgits]

@uvchik what is the difference between changelog and what's new? I don't have a strong opinion on it but if there is no advantage of changelog over what's new I would just keep it as it is.

I will add some information on the weather data, otherwise I think the documentation is fine. I also think the basic usage part of the documentation is fine. What would you add or change?