DOCS UPDATE:Improve installation and environment variable documentation for new contributors

### Is this related to an existing part of the documentation?

- [x] Yes, it is related to an existing section

### What needs to be updated?

## 📌 Current Documentation Issue
As a first-time contributor, I found the onboarding experience unclear.

Currently:
- There is no dedicated installation or setup guide.
- The `.env.example` file lists many environment variables, but:
- It is unclear which variables are mandatory vs optional.
- There are no instructions on how or where to obtain values (e.g., Discord bot token, GitHub token, Gemini API key).

This makes it difficult for new contributors to set up the project locally.

## ✨ Proposed Changes
- Add a basic installation/onboarding guide for first-time contributors.
- Document each environment variable with its purpose and how to obtain it.
- Add a “Common Errors” section to help contributors troubleshoot setup issues.

## 📋 Additional Context

The `.env.example` file exists, but without proper documentation it is hard for new contributors to understand the setup process.
Improving this will reduce onboarding friction and help maintainers by lowering repeated setup questions.

### Relevant Documentation Link (if any)

https://github.com/AOSSIE-Org/Devr.AI/blob/main/.env.example

### Record

- [x] I agree to follow this project's Code of Conduct
- [ ] I want to work on this update