Add installation instructions for Melodic and Galactic

[clalancette]

The front page currently lists Noetic and Foxy, which is great. But Melodic and Galactic are also supported, so we should add these as installation targets.

We might also consider adding Rolling, but since that is a "development" version, we may not want to feature it on the front page.

[kscottz]

While I agree that we should have installation instructions for every distro we have a massive problem with "What version of ROS should I use?" I would imagine that the bulk of the people visiting ros.org are new to ROS; we should guide them towards the simplest solution. We need a more thoughtful approach on how we present this information.