New documentation page + Translation contributions

[Saaaam]

Hello,

Same idea as #3 ; the creation of a page with the same graphical charter, the content will be the same as user_manual_en.md but can be translated depending on the previous language choices made by the user.

So there would be the creation of multiple translation files from user_manual_en.md, such as it is already done with the index.xx.json…