Prepare docs structure to allow multiple languages

[ferborva]

Currently the docs are only written in English and the docs folder structure is setup for only one language. A first nice step to ease the creation of a translated version is to create a /en/ folder with the exisiting docs and update the links.

If you're not sold on the idea of denoting the main language (english) with a new route it can be left as is and have the translations placed inside subfolders. However, if we do have all languages in their own subfolder (/en/, /es/, /it/ ...) we can setup the top-most level for a page that displays links to all available language options.

Thoughts?

[albertogasparin]

Looks like a good idea, even if I wonder if it is a bit premature. Every new language adds some additional work, and I think we should first focus improving/finalising English docs before translating them and having to update the other languages too.