Update and Improve README for CSS Exercises

[Yeetov]

Description:

This PR updates the README with the following improvements:

Clarified instructions for better readability.

Streamlined sections to improve flow and reduce redundancy.

Emphasized the use of Git for version control and discouraged unnecessary PRs.

Improved language consistency and added clear hints for users.

Purpose:

To make the README easier to follow for new learners and ensure better understanding of the exercises and contributing guidelines.

Because

This PR

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

• [ ] I have thoroughly read and understand The Odin Project Contributing Guide
• [ ] The title of this PR follows the location of change: brief description of change format, e.g. 01-flex-center: Update self check
• [ ] The Because section summarizes the reason for this PR
• [ ] The This PR section has a bullet point list describing the changes in this PR
• [ ] If this PR addresses an open issue, it is linked in the Issue section
• [ ] If applicable, I have ensured that the TOP solution files match the Desired Outcome image