docs: enhance CONTRIBUTING.md for new open source contributors

Expands the contribution guide to be more welcoming and comprehensive for first-time contributors.
>
> Key additions:
- Added a 'First-Time Contributors' section with onboarding tips.
- Clarified running, testing, and debugging the CLI locally.
- Documented best practices for template and external generator creation.
- Expanded code style and commit message guidelines with examples.

Author: rahmlad-aramide