[CONTRIBUTING.md] Invalid link in CONTRIBUTING.md

[camillebloch]

The CONTRIBUTING.md file has internal link to the code of conduct which has been moved to seapath_governance.md

[Project Governance](#project-governance)
  * [Project Owner](#project-owner)
  * [Technical Charter](#technical-charter)
  * [Committers](#committers)
  * [Technical Steering Committee](#technical-steering-committee)
  * [Contributors](#contributors)

So the link are no more working. We may make the link to the new file instead