Tutorials section inside st2docs

[arm4b]

One of the ideas and improvements that many people agreed on during the the discussions is the need of "official" StackStorm tutorials being part of the https://docs.stackstorm.com/. There are several StackStorm tutorials in the wild, but no official one yet.

This is something that will help someone new to StackStorm to get started quickly, meet the platform, do meaningful work and achieve real results by following a very simple example.

For reference, Kubernetes Docs has built-in Tutorials section https://kubernetes.io/docs/tutorials/. We could do the same in StackStorm docs.