Add Zenodo badge and DOI to `README.md` and documentation

[jrsmith3]

Overview

I linked this repo to zenodo before making the 2.0.0 release, and now the releases have a DOI (for version 2.0.0 it is 10.5281/zenodo.7030104). The project has DOI 10.5281/zenodo.7030103. Zenodo provides badges for releases like the following.

Deliverables

• Add Zenodo badge (latest) to README.md and the main documentation.
• Add the repo's DOI (10.5281/zenodo.7030103) to CITATION.cff.