sphinx-docs: fix build issues and update ci
Add a job to ensure that the readthedocs documentation remains build-able. Also, pin the versions of all packages used by sphinx to avoid spurious errors due to updates.
GitLab update will be applied on June 28th, between 8am and 10:30am, EDT (UTC-04:00). This site may have intermittent delays during that time.
Add a job to ensure that the readthedocs documentation remains build-able. Also, pin the versions of all packages used by sphinx to avoid spurious errors due to updates.