docs: Add JOSS paper

[rkansal47]

Description

Add paper.md and paper.bib as needed for a JOSS paper submission.
Also adds the Open Journals Github Action to compile the paper.

Action Item Checklist

• Write paper
• Check it compiles
• Review
• Submit to JOSS

Paper checklist

• A list of the authors of the software and their affiliations, using the correct format.
• A summary describing the high-level functionality and purpose of the software for a diverse, non-specialist audience.
• A clear Statement of Need that illustrates the research purpose of the software.
• A list of key references, including to other software addressing related needs.
• Mention (if applicable) a representative set of past or ongoing research projects using the software and recent scholarly publications enabled by it.
• Acknowledgement of any financial support.

[jmduarte]

We should also add the NeurIPS paper to CITATION.cff as a "reference" I guess.