Document how to extract wordings

[matks]

@matthieu-rolland I would like that the knowledge we gathered from the past week issues with the 8.0.0 catalog extraction can be transformed into documentation 😊

It would probably go somewhere here https://devdocs.prestashop.com/1.7/development/internationalization/translation/

Idea is that: all the things you found / discovered / understood from our issues extracting the 8.0.0 catalog : let's write it