Bugfixes and ensuring deployment with stable_baselines1.8 works correctly

Closes #199 (closed) , #197 (closed), #159 (closed)

Breaking Changes:

• Switched requirement for stable_baselines3 to version 1.8. See breaking changes in their releases (1.7 and 1.8!): https://github.com/DLR-RM/stable-baselines3/releases.

New Features:

• Added additional checks to the pipelines to ensure new versions cannot be pushed to pypi while the documentation build fails.

Bugfixes:

• Fixed a bug in deep_mapping_update which prevented loading some network architectures from the configuration.
• Switched back to gym==0.21 and added documentation to aid with the installation process.
• Fixed readthedocs configuration and added julia to the environment to ensure that the entire documentation will be built - up to now it was missing some julia-dependent parts.
• Fixed a bug where the documentation could not be built when julia was installed but pyjulia was built for a different virtual environment. Now the julia installation will not interfere with the documentation build process anymore.
• Updated pre-commit and pipeline (linting) dependencies.
• Changed configuration for test_learning to make it at least a little bit faster.