Note Belle II tutorials and add subsections for different experiments #67

This is a reminder note that Belle II is now giving pyhf tutorials (:exploding_head:) as well. At the 2023 Belle II Summer Workshop Slavomira Stefkova gave a talk on systematics with pyhf and @lorenzennio gave a Belle II focused pyhf tutotrial.

In conversation with @kratsg we agreed that it would be interesting to learn from these tutorials and see if there are components we can pull into the pyhf user guide / tutorial and if there are experiment specific subsections that we could start to add as well.