1580 rework test application using entity class

[leonie19s]

Adresses/Fixes #1580 .

Implements

• the tutorial was shortened to lay the focus more on the CobiGen part instead of Frontend development
• some of the outdated screenshots were replaced
• some of the explanations/steps to follow were rewritten to include more details and to make it more comprehensible
• some steps were modified to make the tutorial applicable to the current version of CobiGen

@devonfw/cobigen