📝(doc) add contributing ladder

[virgile-dev]

So contributors can figure out how to become contributors

General requirements

• I have read and followed the contributing guidelines
• I have read and agreed to the Code of Conduct
• I have added corresponding tests for new features or bug fixes (if applicable)

CI requirements

• I made sure that all existing tests are passing
• I have signed off my commits with git commit --signoff (DCO compliance)
• I have signed my commits with my SSH or GPG key (git commit -S)
• My commit messages follow the required format: <gitmoji>(type) title description
• I have added a changelog entry under ## [Unreleased] section (if noticeable change)

AI requirements

Skip the checkboxes below 👇 If you didn't use AI for your contribution

• I used AI assistance to produce part or all of this contribution
• I have read, reviewed, understood and can explain the code I am submitting
• I can jump in a call or a chat to explain my work to a maintainer