Add a section at the beginning with a link to this project, explaining how to contribute and improve the documentation

### Who for
- Developers
- Potential API doc contributors
### What
- Add a section at the beginning of the published documentation with a link to this project, explaining how to contribute and improve the documentation
- Specifically, a link to https://github.com/openfoodfacts/api-documentation/issues/23
### Why
- Creating a positive feedback and contribution loop

