docs: Add diagrams to Wiki to help visualise Backend system

## Description
As a new contributor of the open source project,
I need to be able to see a visual overview of the existing system that I am going to work on,
so that I can quickly gain an understanding of the system current state.

## Mocks

[draw.io file](https://drive.google.com/file/d/1ELZ4Ym7tCeJXILrcubEFmE717Ns8Zkua/view?usp=sharing)

## Acceptance Criteria
### Update [Required]

- [ ] a diagram with relations between classes, entities, different layers of the application.
- [ ] simplified class diagram showing attributes, methods and/or specific information.
- [ ] Wiki documentation to include the above diagrams

## Definition of Done
- [ ] All of the required items are completed.
- [ ] Approval by 1 mentor.

## Estimation

1 hr