More examples of custom Python plugins

[DoeringChristian]

The documentation shows how to Implement custom python plugins in the example of implementing a BSDF. This is a great starting off point for writing your own BSDFs but it is not always that easy.
For example, implementing an Integrator (see #166). It might be useful for people new to mitsuba3 to have examples of such implementations available as tutorials similar to the one about implementing a BSDF.
Maybe a tutorial building on top of Scripting a Renderer implementing mitsuba3's classes could be interesting.
This way someone having learnt about ray tracing and knowing the basics could then learn how to use the infrastructure mitsuba3 provides to implement their ideas in a more idiomatic way.

Thanks for any ideas or comments regarding this request.