Add 1.2 Beta docs and download to website

Nadie ha tomado este issue todavía.

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

Empieza revisando la guía de referencia de la documentación del sitio web, así como la navegación de versiones y el estilo de las páginas existentes. Implementa el selector de versiones y las advertencias indicadas para stable, dev, outdated-version y work-in-progress; se considera terminado cuando se pueda acceder a cada versión como se describe y muestre la advertencia correspondiente.

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