Publish site via GitHub Actions

[bennibbelink]

Closes #344. Instead of maintaining a main branch to publish the site this will publish the site via a GitHub workflow on any merge to source. Attempting to follow best practices outlined in GitHub documentation.

[bennibbelink]

Comment from the linked issue:

That said, this relies in our custom build docker images having the right config. We probably want that to be part of the workflow somewhere? I'm not sure all the images have Sphinx. I saw that install in your sample build, but I didn't really see where that happened in your sample PR.

I updated docker/fuelcycle.org-deps/Dockerfile to build FROM our latest cymetric image and install Sphinx. This should ensure that all of our cyclus libraries exist in the docker image. I also modified the docker-gh-preview docker-html targets to docker build using the Dockerfile. Thus in the workflow we just need to call make docker-html to build the site.

[bennibbelink]

Another advantage of this moving forward is that to publish the site when we make changes to the cyclus/cyclus repo we need only run the workflow via workflow_dispatch (for example after we update the dependencies in the cyclus/cyclus documentation)

[gonuke]

LGTM - just one question about the them pinning

[gonuke]

Is this pinning to preserve the sidebar menu? If so, let's add an issue to update our templates for a newer theme version.

[bennibbelink]

I am not sure why it is pinned at 1.6, that's just what was there. I'll make an issue regardless and try to resolve the sidebar complications.

[gonuke]

If it's not for any reason, maybe let's drop the pin and see what we get?

[bennibbelink]

I dropped the pin and don't notice any significant changes (if any at all). If you want you can download the artifact generated by the action and check it out yourself! 😃

https://github.com/cyclus/cyclus.github.com/actions/runs/8461607773