add order of translation formats

The preferred order of the different translation formats in
configuration blocks is Xliff, YAML, PHP. Thus, this has to be
documented in the contributing section. Since the order being used
before was Xliff, PHP, YAML, existing examples had to be modified
accordingly.
minor #4112 [Contributing][Documentation] add order of translation formats (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Contributing][Documentation] add order of translation formats

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
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ef4dd23 add order of translation formats