REPEAT: Implement Suggested Feedback and Create Katacoda Version of Tutorial

[felix-seifert]

According to your feedback email about our tutorial about serveless applications on Heroku in the original PR #1215, @Ackuq and I implemented the suggested feedback in different PRs which are referenced in the following list.

• Executable: To call the URL with the logs successfully, it has to be surrounded with quotation marks. To avoid confusion whether you should copy the URL with the quotation marks or not, we added the hints in to the tutorial.
• Tutorial on online platform: If readers do not want their own infrastructure to influence the outcome of the tutorial, they can perform a slightly adapted version of the tutorial on Katacode. The Katacoda tutorial is also referenced in the original GitHub tutorial.
• Describe Heroku CLI more clearly: We described the Heroku CLI and Heroku's Platform API briefly and separated them more clearly.
• Describe Dynos and One-off Dynos: We also made a clearer description of Dynos and the more specific One-off Dyno. The motivation to use them is then implied by the motivation of the whole tutorial.
• In addition, we revised and improved some steps which happened scattered throughout the different PRS.