Remove duplicates in citation information

[BenjaminRodenberg]

When going through the repository I found another point that should potentially be discussed: Where/How do we want to maintain the citation information of the FEniCS adapter.

Currently, we have to maintain three places:

1. This CITATION.cff file.

   The file and citation mechanism might be new to many people so we might need to explicitly mention GitHub's citation mechanism somewhere.

2. There is a citation section in the README.

3. There is a section on the adapter website that is part of the project.

Originally posted by @ajaust in #157 (review)

[BenjaminRodenberg]

For putting this into context: We are currently starting to find out how we can use CITATIONS.cff for a project like preCICE (see also precice/precice#1703). Therefore, I would not prioritize reaching a 100% consistent and duplicate-free status at the moment (should be our final goal, of course), because this would mean that we have to roll this out for all adapters. I still decided to open this issue, because the comments above are all valid and we should try to address them at some point in the future.

Additionally, there is also the list of contributers provided by github.