Blog post introducing the Scala Installations feature.

This documents https://github.com/scala-ide/scala-ide/pull/731

The PR is going out early (see tasks below) for review purposes : at .2.5K words, it's a hefty read.

- [ ] Add a banner on the old multiple versions blog posts saying the present post supersedes them.
Hm, the `~` is a bit confusing. Is that intentional?