doc: Only build HTML doc in ReadTheDocs
This should make the build process faster since we seem to be hitting some timeout.
GitLab Maintenance scheduled for Thursday, 18th December 2025, from 7:00 AM to 10:00 AM BST. GitLab will be in read-only mode and experience short periods of complete downtime (up to 20 minutes) during this window.
This should make the build process faster since we seem to be hitting some timeout.