Improve contributing guidelines

[JeanChristopheMorinPerso]

We need to document our current development practices, enforce tests in PRs, etc.

Requirements:

• https://www.bestpractices.dev/en/projects/8389#contribution
• https://www.bestpractices.dev/en/projects/8389#contribution_requirements
• https://www.bestpractices.dev/en/projects/8389?criteria_level=1#coding_standards
• https://www.bestpractices.dev/en/projects/8389?criteria_level=1#coding_standards_enforced
• https://www.bestpractices.dev/en/projects/8389?criteria_level=1#test_policy_mandated
• https://www.bestpractices.dev/en/projects/8389?criteria_level=2#code_review_standards