docs: add CONTRIBUTING.md

[LesnyRumcajs]

Summary of changes

Changes introduced in this pull request:

• added CONTRIBUTING.md with some help from the https://generator.contributing.md/ (attributed at the bottom),
• removed the old contributing.md

Reference issue to close (if applicable)

Closes #4847

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.