v2: Add how-tos

[dweindl]

I think it would be helpful to have some how-tos for addressing common application problems in PEtab (in additional to the basic tutorial).

Some cases that come to mind:

• Dealing with relative measurements (-> scaling parameters)
• Batch effects (-> condition-specific observable/noise parameters)
• Use of multiple models and model-specific parameter values (v2: Clarification MultiModel format #622 (comment))
• ...

The simple cases can probably also be addressed in some FAQ section.

[sebapersson]

I fully agree. I think a good structure would be to have the core tutorial, some of the more advanced features as extended tutorials (expanding the core tutorial), and the more simple things as FAQ.