Add "learning goals" to each recipe

[Pfeil]

While the title often defines the learning goal very well, sometimes it might make sense to specify the goals explicitly.

Example: The recipe "Validate a PID record" could have the following goals:

• Be able to understand what validation means and which kinds of validations exist.
• Be able to apply existing validation implementations (currently not really included, as there is no API description or example, but only mentioned that the Typed PID Maker can do the basic validation).
• Be able to implement validations.

My pro argument: This does not require the reader to see themself as a "data curator" to recognize this text is suited (avoids "target group statement per recipe")

Structure proposal:

Current recipe structure	Proposal
Title	Title
	Learning Goals
Introduction / Summary	Introduction / Summary
Ingredients	Ingredients
Step-List	Step-List
Step-Instructions	Step-Instructions