Explain graphically the relation between `sbi`, `pyknós` and `nflows`.

[alvorithm]

Create a block diagram and associated caption explaining the relationsip between the three packages.

[jahma]

yes yes yes we want this-- looking forward to seeing a first draft (this can be done quickly, btw, no need for super fancy graphics or tikz-whatever)

[jan-matthis]

(Probably not needed anymore once we migrate to flowtorch.)

[janfb]

related to #336