Test docstring examples

[hakonanes]

Description of the change

• Add a build step to test the docstring examples using doctest via the pytest command pytest --doctest-modules.
• Update pytest config accordingly.
• Fix docstring examples accordingly.
• Update contributing guide with tips for running docstring tests locally

Progress of the PR

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black via pre-commit

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the
  unreleased section in doc/changelog.rst.