Improve the documentation

[salmma]

regarding: https://qhana.readthedocs.io/en/latest/index.html

please describe:

• the purpose of QHAna
• what an experiment ist
• for what the plugins are, why I should use them, how they are related to an experiment
  such that a user who uses this tool for the first time has a better understanding of the tool and the different steps and make it more informative.

as eg described here https://quantil.readthedocs.io/en/latest/user-guide/qhana-new/

Is this https://quantil.readthedocs.io/en/latest/user-guide/qhana/ still required? If not please remove.

If QHAna should have a separate documentation then I would suggest to remove the detailed description from the QuAntiL documentation and just share a link to the QHAna documentation - to keep consistency - equivalent to the PatternAtlas (see landing page of the QuAntiL documentation).