read the docs check/build actions

[robfitzgerald]

currently, we build our readthedocs content when testing a pull request. this means we push changes to readthedocs any time someone updates a commit on an active PR. since this is now an open source project, this means that a random github user could submit a PR with changes to our documentation and it would run the docs build process for them.

proposed change:

1. copy the build action over to a push action (based on this comment)
2. if sphinx supports a dry-run operation to test the validity of the docs: (on a cursory search of the docs i didn't find one)
   • change the current PR action so that it executes a dry run of building the docs, which at least confirms the validity of any docs changes

[nreinicke]

We should confirm this, but I'm pretty sure that the PR action only pushes the documentation changes to a staging site, see here: https://nrelhive--143.org.readthedocs.build/en/143/ and not to the main documentation site until the PR is merged.