Improve markdown files content validation

[yannbertrand]

trafficstars

Switching from defaults.yml to markdown files removed the automatic content validation rules we had. (see yml schema and tests)

We could improve validation for contributors and help maintenance. Here are a few things I have in mind:

• [ ] Improve contributing documentation
• [ ] Create a markdown template file that contributors could copy
• [ ] Validate content in CI
  • Maybe using Marked?