Update CITING.md with new papers

[pratikvn]

Following the suggestion from @tcojean, this PR adds new papers and sections to the CITING.md file

[tcojean]

Thanks Pratik for taking this up! I agree with Marcel, we should not clutter this file too much. The main point from my PoV is to concentrate citations on e.g. journal papers rather than have a few citations spread over too many papers. It's up to everyone to pick which papers they prefer is cited for their research, but I do think we should keep this list as short as possible.

[pratikvn]

I think that is quite difficult to do. In many cases, papers are mostly intertwined with applications, so specifying one paper for the feature is difficult. I mostly added a few main papers for each, and let the user decide which they want to use. Maybe we can make a rule that we add two papers per section to keep this file limited.

I think maybe one approach would be to have a recommendation only for the main Ginkgo paper, but list maybe two/three papers for each major feature, and let the user decide if they want to cite something additional to the main Ginkgo paper.