Add “vision” section to community page

[miri64]

Now that our vision page was moved from the RIOT wiki on GitHub to the in-repo documentation, we could just mirror it from the RIOT repo by including _RIOT/doc/doxygen/vision.md, as proposed by @waehlisch in #124. Currently, our vision is just linked in the RIOT History on the community page. One idea for including vision.md is to add a new section to community.html and just link that in our History section.