Update gh-pages with a circleci job

[jdahm]

Purpose

Updates the documentation on the gh-pages for every merge to main.

TODO:

• Check that this works
• Add credentials
• Ensure it only executes on commits on main

Infrastructure changes:

• Adds a new CircleCI job.

[mcgibbon]

Can you merge this plan with the doctest plan? These should only be pushed if doctests pass anyways.

[jdahm]

As far as I can tell these two plans are doing different things, and should be run at different times, so I'm not sure what commonality there is to merge them. The new plan push_docs builds and pushes docs when a PR is merged to main, while the doctest plan tests that the sphinx docs build and the snippets run without error on any change to a PR.

[jdahm]

Right, this will only run if the doctest plan passes, since that is a branch protection requirement. However, we don't want this running every time the doctest passes, since that would mean changes in PRs affect the "prod" instance of the docs.

[elynnwu]

lgtm