Docs, or part of Docs, Should Only Generate on Release

[markpbaggett]

It just occurred to me that docs are always regenerated on merge. This is problematic since we don't publish to py.pi on merge. We should think about this going forward.

[glenrobson]

Change https://github.com/iiif-prezi/iiif-prezi3/blob/main/.github/workflows/docs.yml to run on publish rather than merge.