Document the system overview and architecture

[firasrg]

Create a detailed system overview for the Car Service System, outlining the WHAT, WHY, and HOW of the project.

Break down the system into main components and subcomponents, and set up an entity modeling system using UML. Include diagrams such as use case, activity, class, sequence, and state (optional) to visualize the architecture.

Note⚠️ :This issue is not about writing code; it's about thoughtfully planning how the system of this project should be structured before any code is written. While coding is the easy part, the real challenge lies in designing the system's architecture. While there is already some code written, it might be fully reviewed when this issue is done.

This issue is highly attached to #6