Add general app documentation

[jpwahle]

Is your feature request related to a problem? Please describe.
Each repository has API documentation (e.g., backend, prediction-endpoint). To get users started more general, we should have general documentation and link it to the main repo's readme.
The docs should be here and also linked to the cs-insihgts-main repo.

Describe the solution you'd like

• Write general docs. Also, link technical documentation of each project whenever appropriate.
• Link to frontend whenever we have a "documentation" link.
• Link documentation in cs-insights-main readme.

[jpwahle]

Although very important, I will move this towards the end of the advanced research project from @muhammadtalha242 to avoid double work and having to adjust the docs many times.