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

首先查看网站的文档参考指南，以及现有的版本导航和页面样式。实现版本选择器，并实现针对 stable、dev、outdated-version 和 work-in-progress 的所述警告；当每个版本都能按描述访问并显示相应警告时，即表示完成。