Added CITATION.cff file and set up Zenodo DOI registration (external)

[Luna-Klatzer]

What type of change does this PR perform?

• [x] Info or documentation change (Non-breaking change that updates repo info files (e.g. README.md, CONTRIBUTING.md, etc.) or online documentation)

Summary

Added CITATION.cff to allow others to more easily reference this project and also get the important metadata.

Closes #500

List of Changes

• Added CITATION.cff.

Does this PR create new warnings?

None.

Detailed Changelog

None.

Linked issues or PRs

• [x] #500

[github-actions[bot]]

size-limit report 📦

Path	Size	Loading time (3g)	Running time (snapdragon)	Total time
./kipper/web/kipper-standalone.min.js	128.56 KB (0%)	2.6 s (0%)	1.5 s (+33.71% 🔺)	4.1 s
./kipper/web/kipper-standalone.js	128.6 KB (0%)	2.6 s (0%)	1.2 s (-2.21% 🔽)	3.8 s