Create Basic `FastAPI` Backend Example

[xKabbe]

Description

We choose a https://github.com/xKabbe/star/labels/python backend by using FastAPI. To get started with this, we want a basic setup of the backend and a first "Hello World"-API endpoint.

Additional Context

• FastAPI

Expected Actions

• [ ] Set up the FastAPI backend
• [ ] Create a first "Hello World"-API endpoint
• [ ] Document how to set up and start the backend itself
• [ ] Document how to check the endpoint documentation (swagger)
• [ ] Create a first backend unit test and add a backend workflow for https://github.com/xKabbe/star/labels/github%20actions

Definition Of Done

• [ ] Code implemented
• [ ] Tests implemented and passing
• [ ] Documentation implemented
• [ ] GitHub Actions are passing