make it possible to update documentation without cutting a new release

I've been doing a lot of documentation updates between releases that would be useful to deploy. Our current docs build depends on a tag being pushed and [the docs step running](https://github.com/entropic-dev/boltzmann/blob/latest/.github/workflows/build.yml#L121). If we pull the docs step into a different action with different triggers, however, we might be able to update the deployed docs on editing a release, say.