Rewrite the tutorial "My first content app".

Compared to the old version - which is done with a Magnolia Maven module, the new tutorial will be rather different. Here are some key points of the new approach:

• All is done within a light module.
• The "basic" version must utilize a Content type definition
  • e.g. reuse the app tourGuides-app.
• An extended version maybe could be done without a Content type definition (not sure about this) ...
• We could reuse sample code which is already used for the Content type docs (Add link here to the final destination of these examples)
  However - the CT sample code light module contains already 4 CTs and 4 apps.
  For this tutorial it could be more meaningful to have something distinct which resides in its own light module.
  But the existing examples can be used for inspiration.
  ¯_(ツ)_/¯

 This app as shown in this tutorial will be the base for some other tutorials, which must be rewritten, and which earlier have been relying on the old version of the "My first content app". That's why this story has a high prio.