Update DGE chapter

[LuisHeinzlmeier]

• remove single-cell specific DGE (only keep a short text/explanation)
• replace custom aggregate_and_filter() function with scanpy.get.aggregate() (preferred) or decoupler
• use PyDESeq2 to replace the R code if possible (pertly.tools could also help)
• include new figures with pertpy
• add a small dropdown in case the reader only has one technical/biological replicate

[LuisHeinzlmeier]

@Zethson, feel free to add anything I may have missed.

[Zethson]

That's great, thank you!