Add the Zenodo DOI badge

[vahid-ahmadi]

The v1.5.5 release is archived on Zenodo, but nothing in the repository says so.

Adds the badge beneath the existing Build and Codecov ones, using the concept DOI 10.5281/zenodo.22829459 rather than the version DOI 10.5281/zenodo.22829460, so it always resolves to the latest archived version instead of pinning to v1.5.5.

Both the badge image and the DOI redirect return 200.

The README is also the PyPI long description, so the badge appears there at the next release. The DOIs are already recorded in CITATION.cff on the joss-paper branch (#315).

[juaristi22]

Checked every claim live. The badge SVG returns 200. The concept DOI 10.5281/zenodo.22829459 redirects to the record for v1.5.5, which is the latest archived version, so it will keep resolving to the newest release; the version DOI 10.5281/zenodo.22829460 resolves to the same record. The record's title and four authors match .zenodo.json from #328, it is linked to the v1.5.5 tree, and CITATION.cff on the joss-paper branch cites the same concept DOI.

One thing worth knowing for the follow-ups: the v1.5.5 release behind this record was created by hand. Tags pushed by the versioning workflow do not create GitHub releases, so a new Zenodo version will exist only when someone creates the release. Approving.