Add template: OEP-design document

Description of the issue

The oep is constantly growing, but it still lacks a definition for design documents that describe its functions and create commonalities. This documentation is very important considering the long-term development and complex functions. The would be created for each feature and should explain the process in plain English and then go into technical detail by describing the process and technical implementation as well as the infrastructure (software libraries that are used or services that need to be set up).

There are a lot of templates for design documents that we should review. I suggest a draft of the template below.

Ideas of solution

I suggest a draft that IMO can already be used to create dsign documents for OEP features:

Note: I would not require a design document to be created for all functions, but only for those where the implementation is not immediately clear and where a lot of knowledge is only available to 1-2 developers.

# Feature design document

## Abstract

## Limitations

## Prozess

## Feature description & Components / Software structure

## Testing

## Infrastructure updates & tasks

## Technologies & Purposes

## Release Plan für feature

## Workflow checklist

• I am aware of the workflow in CONTRIBUTING.md