Add quality requirements for AppCat

[Kidswiss]

Adds various quality requirements to the AppCat Framework.

Format is inspired by https://github.com/appuio/appuio-io-docs/

[ccremer]

Overall a good starting point 👍
This PR has gotten very large and I didn't fully review it yet.
Might consider splitting it up in some way.

Reading through it could roughly group the requirements into 2 categories:

• The ones targeting the AppCat as a Framework itself to establish a base standard. (e.g. some in usability or regarding monitoring)
• The ones that are try to dictate how services themselves should behave or be engineered. (e.g. security, performance)

I think it's better to only list the ones directly applicable for the framework. And then each service should be treated independently including things like security or performance aspects.

[ccremer]

I feel these discussions are somewhat inefficient but very important. Let's make a call to discuss the details and then do some pair programming to get this done.

[Kidswiss]

I've rephrased the points that were still open. We can take that as a starting point for a call.

[ccremer]

@Kidswiss and I had a call to flesh out duplicates and things that are better left documented in the future service development guide. This PR got smaller and more structured.