docs: clarify how to cite PySDM (papers + zenodo)

[olastrz]

there is a hint in README:

https://github.com/open-atmos/PySDM/blob/ea9144e0d21dea7e17b01dec4fd9ce38d4a5062f/README.md?plain=1#L62-L64

but nothing in the docs, for an example way to include it in the docs, see https://open-atmos.github.io/PyPartMC/PyPartMC.html#acknowledgement-and-citations

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