Add a documentation page about campaign actions
Created by: mrnugget
This adds a new documentation page under "Campaigns", called "Actions" to explain
- What actions are and what they're used for
- How to define them and what different types of steps are
- How to execute them
There's still a bit of duplication between this page and the other pages, but right now I don't think duplication hurts. We can then always remove it once we refresh the other pages.