Update CI script

Description

The CI script has been updated.

• added new job that tests documentation for non-master branches
• sphinx-build now fails on warnings (required some small adjustments to documentation)
• added xdoctest to the CI (required some small adjustments to existing code examples)
• minimal type hinting in force_constant_model.py
• updated Docker image (including update to sphinx version 3)
• dropped support for Python 3.5