Add tutorial for creating automatically xlf translation file - Update new-system.md

[vcopleutre]

For new module developpers, one of our questions is "how to create the translation file".

This part add the best practice (according to my own judgement) do to it.

The syntax may not be efficient, feel free to reword it or add internal anchors to the text.

Questions	Answers
Branch?	8.x
Description?	Answering the question for developpers : "how to create automatically the xlf translation file in my module"
Fixed ticket?	none

[thomasnares]

Thanks @vcopleutre, i added a few suggestions with internal linking 👍

[kpodemski]

thanks! a few comments and it's all good :)

all good

[kpodemski]

thanks for contributing @vcopleutre !

[vcopleutre]

Thank you for the updates!