Add links to JOSS paper

[ortk95]

Once the JOSS paper is accepted & published, add relevant links to it:

• [x] Readme (badge & explicit section)
• [x] Main documentation pages
• [x] Source code (init docstring & maybe any module level dunder constants)
• [x] CITATION file
• [x] setup.py urls
• [x] ? Can maybe add link to .zenodo.json

https://github.com/openjournals/joss-reviews/issues/5728