Revise CI

• Slimmed down Docker image to under 200 MB (previously over one GB). The Docker image now only contains packages needed for compiling the documentation.
• Revised the CI
  • used slim python base images for most jobs (avoids downloading the custom image at the cost of having to install some packages from scratch)
  • added a job that allows running the pages task for MRs and branches manually to enable testing before deploying
  • removed the flake8 job for master and schedules
  • converted legacy only/except to rules
  • remove code quality test (takes a lot of time and is not used)
• fixed doctest in units module