[Contributing][Documentation] add order of translation formats

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 
@WouterJ soon we'll explain them in detail, but for now: 1) broad support among tools used by professional translators; 2) validation (thanks to XML); 3) easy to look for translation keys (hierarchical YAML files make this "impossible").