Update citation info and maintainer info

[PhilippBach]

Description of the issue or change proposal

• Update citation info for API according to Add a CITATION.cff to the Repo doubleml-for-py#117

• Also remove Malte from readme / index page , fixed in 84bfda6

Comments, context or references

No response

[SvenKlaassen]

@PhilippBach In which form do you want to add the citation?
Just to the README.md and the main page https://docs.doubleml.org/stable/index.html#citation ?

[PhilippBach]

I think I'd add it here

https://github.com/DoubleML/doubleml-for-py?tab=readme-ov-file#citation

and

https://github.com/DoubleML/doubleml-docs/blob/main/doc/index.rst#citation

[SvenKlaassen]

@PhilippBach
The README has already a small text:

"You can click the "Cite this repository" button at the top of the GitHub page to cite the package directly. Alternatively, you can cite the following paper:"

But we could exchange the bibtex entry.