Add longer/shorter examples section to docs, restrict Sphinx version

[hakonanes]

Description of the change

• Add shorter and longer examples section to the documentation. I decided this was the best place for the M&M 2021 Sunday Short Course notebook (original notebook).
• Add a short example showing how to save a master pattern in the stereographic to an image file without a background (I frequently use this script myself).
• Restrict newest Sphinx version to 4.0.2 to fix Doc build failing due to known issue #401.
• Move all user guide notebooks to a doc/user_guide directory. This means all URLs will be "https:/ /kikuchipy.org/en/stable/user_guide/load_save_data.html" and similar, where the "/user_guide" is new. This is done to add some order to the doc/ directory.

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.