Improve CONTRIBUTING.md documentation

**What**
- Adds linux and macOS instructions in CONTRIBUTING.md's "Development Environment Setup" section
- Adds video and screenshots to make the process easier to understand

**Why**
- Current document does not provide instructions for linux or macOs
- Also does not provide enough explanation in the steps
- Lack of detailed instructions hurts the onboarding process for new members

**How**
- [Flask tutorial](https://flask.palletsprojects.com/en/2.1.x/installation/)
- [Video on setting up application on windows](https://drive.google.com/file/d/1DAtgfW_B5ObfO0uKejvPCoJbrGXlwpZq/view?usp=sharing)

**Requirements**
- Improved steps should be clear and descriptive
- There should be visual elements (screenshots or video)