Rework "Getting started" tutorial to create a workflow

We hadn't seriously revised this tutorial since adding the debian_pipeline workflow, which is now a much better thing for people to run on a source package than a simple sbuild work request.

I moved some of the preparatory steps to a separate "Set up workflow templates" tutorial, and added debian_pipeline to that. I also adjusted the tutorial to be clearer about what you need to do differently on a local instance vs. on debusine.debian.net.

Fixes: #764 (closed)