Add 1.2 Beta docs and download to website

I like what Django is doing: https://docs.djangoproject.com/en/1.8/intro/overview/#install-it

• Have a selector overlay allowing to switch between versions
• on the main page (http://mapstruct.org/documentation/reference-guide/), there should only be a link to "stable" and "dev", pointing to the current stable and work-in-progress versions
• on older versions (as reachable through that selector) there should be a red banner of sorts warning about this being not the current version
• on the dev version (as reachable through that selector and from the main page) there should be a yellow banner warning about this being a future work-in-progress version

A useful resource: http://webmasters.stackexchange.com/questions/99867/how-to-correctly-mark-up-different-versions-of-the-same-document-which-are-non-c

Commencez par examiner le guide de référence de la documentation du site web, ainsi que la navigation entre les versions et le style des pages existants. Implémentez le sélecteur de version et les avertissements indiqués pour stable, dev, outdated-version et work-in-progress ; le travail est considéré comme terminé lorsque chaque version est accessible comme décrit et affiche l’avertissement approprié.