[CI] Have GitHub Actions export built website to somewhere upon success

[myShoggoth]

This could be to a different repo, a different branch, a submodule, some shared location that's easy to get to.

Ideally it would be a live integration server where we can just see the current version of the main branch.

[myShoggoth]

Ideally the main branch's CI would release to the production website, while any other branch would have its own (location easily determined based on the branch name) test website.

[myShoggoth]

Some commentary in this PR: #101 (comment)

[myShoggoth]

This is partially done, the hakyll branch, when built, does export the site to the main branch, which is then live. However, we do not currently have a staging area for PRs / other branches.

[visortelle]

Working on it.