Add explanation of the different abstraction levels

[greschd]

Description of the modifications

Add a "explanations" section (following this schema, name TBD) to the docs.

One thing that would warrant explanation is the different abstraction levels, high-level CompositeModel vs. operator-oriented. This could be linked to for example from the Material Properties example, where operators come out of nowhere.

If the operator concept is already well-explained in the DPF docs, we can also just put it in context and link there.

Useful links and references

https://documentation.divio.com/