Add getting started for cd generator

[Hendrik7889]

I also edited the cdlang\src\main\java\de\monticore\cd\codegen\decorators\AbstractMethodDecorator.java to now include generation errors when a non-abstract method is not implemented using the TOP-mechanism.
Furthermore, the PrettyPrinter option has been added to the cdgen tool.

[github-actions]

✔️ Changes pass the MontiVerse

Project	Status

details

Project	Status
montitransruntime	⏩
montiarc	✅
umlp_experiments	✅
umlp_tool	✅
sd	✅
montifun	✅
fd	✅
automaton	✅
sc	✅
guidsl	✅
artifacts	✅
ocl	✅
od	✅
ucd	⏩
langexperiments	⏩
json	⏩
xml	⏩
cd2gui	✅
mlc	⏩
monticore_future	✅

The MontiVerse is a collection of (internal and public) language projects.