DOC: découpage de la documentation en plusieurs pages

doc/en/how-to-translate-the-website.md

+How to translate the website
+===
+
+## How to translate the main contents
+
+For pages such as:
+
+- Home
+- Features
+- Download
+
+..and header and footer menus, 
+
+the changes take place in *.po files. 
+
+You will find those files in folders such as `i18n/fr_FR/LC_MESSAGES/` .
+
+Such files can be edited with softwares such as [PoEdit](https://poedit.net/).
+
+## How to translate the tutorial
+
+To translate the tutorial, you will find .html files in folders such as `i18n/fr_FR/contents/tuto/`, where `fr_FR` is the language code for the French (fr) variant that is spoken in France (FR).
+