Google Actions CI: documentation testing

[rwest]

The documentation testing script at https://github.com/ReactionMechanismGenerator/RMG-Py/blob/master/.github/workflows/docs.yml checks for broken links in the documentation in the "Check documentation links" step. A recent build spent about 6 hours here before the overall job timed out and was killed. We should add a timeout on that step somehow, or as an option to sphinx-build if there is one, so we don't waste so long of Github Actions cpu time.