Add `CITATION.cff` file to support native citation feature of GitHub

[JonathanWenger]

Is your feature request related to a problem? Please describe.
Currently there is not consistent way to cite ProbNum.

Give an example use case.
If somebody is using ProbNum to compare their method against (e.g. this paper) or if somebody implements their method in ProbNum, there is currently no unified way to cite the library.

Describe the solution you'd like.
Add a CITATION.cff file such that a bibtex entry can be exported directly from the main page of the repository, like so:

Additional context
This issue may be blocked by a necessary authorship discussion.