Restructure docs into Tutorials, Examples and improved API reference

[hakonanes]

Description of the change

See #372.

Progress of the PR

• [ ] Docstrings for all functions
• [ ] Unit tests with pytest for all lines
• [ ] Clean code style by running black via pre-commit
• [ ] Complete touch-ups of tutorials (old user guides)
• [ ] Move some parts of tutorials to examples (Wullf net, symmetry elements in stereographic projection, things like this more suitable as examples to make the tutorials more focused)
• [ ] Fix API reference (update docstrings, module docstrings etc.)
• [ ] Fix docstring examples so they all work as written (tested using doctest)
• [ ] Update contributing guide

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 CHANGELOG.rst.
• [ ] Contributor(s) are listed correctly in __credits__ in orix/__init__.py and in .zenodo.json.