Make intro page fit new curriculum

:construction:  I'm opening this PR so we can get early feedback. It's not ready to merge! :construction: 

The [Javascript curriculum](https://github.com/zspencer/snake-game-tutorial) created by @zspencer and @zassmin had it's trial run. (And there will be others issues and pull requests for improvements.)

But it's probably useful to pull the curriculum into docs so we can get more eyes on it.

If anyone wants to help, we need to take over the stub of a javascript curriculum currently in docs (https://github.com/railsbridge/docs/tree/master/sites/en/javascript)
and pull over the content that's currently in Zee's repo. (https://github.com/zspencer/snake-game-tutorial)