Improving userguide introduction

[IAlibay]

cc @hannahbaumann

The userguide introduction is very specific to the idea of running a "workflow", and doesn't necessarily provide a lot of information about what else is in the userguide. From an initial look, it might be hard to know what to expect from the userguide from that page.

The general idea of "here are the main steps of my workflow and this is what the userguide breaks into" makes sense, but it might need some padding / restructuring?