ci: Build and push docs to Github Pages

[langston-barrett]

Rather than ask interested parties to build the documentation themselves, we could build it in CI and host in on Github pages. See this build for an example. This should be done after open-sourcing, as the Github pages are public by default. This will require a repo administrator to enable Github Pages in the repo settings.

[kquick]

FYI, a more complicated CI process that allows for publishing multiple versions can be found here: https://github.com/GaloisInc/cryptol/blob/master/.github/workflows/docs.yml, and rendered here https://galoisinc.github.io/cryptol/master/RefMan.html (see the "Doc version" selector at the bottom left).

That may be overkill for current needs, but this allows for user selection of a version (with a supporting selector) including active PR's in case there are multiple release versions.