Feature/update docs

[MaGering]

Resolves #81

With this PR the docs are updated with further information on:

• Setting up scenarios
• Convention on oemof-B3 _resources and model-own assumptions
• References to oemoflex

It needs https://github.com/rl-institut/oemoflex/pull/83 for better understanding.

[jnnr]

Hi @MaGering, I updated #270. Can you merge the respective branch here and resolve the conflicts?