Add planning question on public transport to the "isochrone" tutorials

[UJehle]

Goal of this issue

In the tutorials, currently, a section on the new public transport analysis is missing. https://plan4better.de/tutorials/isochrone/

Add a step-by-step description + screenshots on the planning question "How many people can access a specific location within 20min by rail-based public transport (tram, underground, train)?"

The result would roughly look like this: