doc: Make conf.py friendlier to RTD
Since RTD fails to import lisa for unknown reason, even if it shares the
same venv, move the sys.path update before import lisa.
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.
Since RTD fails to import lisa for unknown reason, even if it shares the
same venv, move the sys.path update before import lisa.