Set up the boilerplate and initial content for the Use Case Portal

[jgeluk]

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[jgeluk]

@rivettp @DennisWisnosky @FroehlichMarcel

Wanted to make a start with the Use Case Portal as envisioned years ago in the early days of the
EKGF and do it via 1 git repo (for now) where we can collect all use cases that we can think of that
are right (or not right) for the EKG, document them, come up with a sort of template for a use case
and continuously improve until we find the right structure for it.

Eventually I think that every use case should be in its own git repo but for now we can do the "monolithic approach" :-)

[rivettp]

@jgeluk do we have a draft template for a use case?