Doc: Document Python dependencies in the Quick Start Guide

[Paebbels]

The Quick Start Guide lists Python 3 as a dependency, but it misses a list of Package dependencies, which could be installed from PyPI.

Work Items:

• Check if the Quick Start Guide links to the full documentation.
• Check if the full documentation contains all Python dependencies.
• Add a hint how to fulfill all Python dependencies via pip.
• List Python dependencies for other flows:
  • Appveyor
  • ReadTheDocs
  • Travis-CI
• Register PoC at Requires.io to check if Python dependencies are up-to-date.