Bug in docs

[Irieo]

we have a broken documentation in several places:

https://pypsa-eur.readthedocs.io/en/latest/preparation.html#module-simplify_network
https://pypsa-eur.readthedocs.io/en/latest/preparation.html#module-cluster_network

the problem is in path to exemplary images: https://github.com/PyPSA/pypsa-eur/tree/master/doc/img

Tracing back file change history, the bug was introduced here #1201 on 13 Sep 2024, possibly due to wrong find-and-replace action or LLMs.

A fix would be updating doc strings everywhere, and checking that documentation is compiled right.