first draft PR docs build workflow

[briantist]

This PR adds a workflow that allows for building documentation in PRs. We already have publishing with RTD for releases (tags) and pushes to main, and we have a CI job that runs doctest (which implicitly also ensures that docs are valid since they need to be built).

This workflow is a little different. On a PR, it builds the HTML docsite for both the PR contents and for the target branch (for example main), and then diffs them. If there are any changes, it:

• publishes a temporary version of the docsite on GH pages (so PR authors and reviewers can see it rendered)
• posts a comment to PR that contains the list of changed files, diff output, and links to the rendered versions

This workflow uses the pull_request_target trigger which has a few caveats and things to know:

• the workflow itself is always loaded from the PR's target branch (like main), so the workflow that gets executed is never from the PR itself (this is a necessary security decision)
• development of the workflow (like in this PR) then is a little tricky:
  • the PR branch should be pushed up to this repo, and not to a fork (so it must be done by a maintainer)
  • then, you can open a new PR (this one can be from a fork) against the first PR's branch as the target, and that will allow the "modified" version of the workflow to run
  • the test PR can be closed when it's done
• because a PR author without write access can't modify the workflow, that also means that on PRs from new users, these workflows run without someone needing to approve them, which is nice since the feedback is fast

[codecov]

Codecov Report

Merging #1042 (aa7c063) into main (8e2007e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1042   +/-   ##
=======================================
  Coverage   85.00%   85.00%           
=======================================
  Files          65       65           
  Lines        3135     3135           
=======================================
  Hits         2665     2665           
  Misses        470      470