Improve documentation

[hakonanes]

Description of the change

This PR improves the documentation in the following ways:

• Make API reference comply with numpydoc
• Restructure documentation into three distinct learning resources: Examples (new), Tutorials (previous user guide), API reference. This structure is inspired by Diátaxis (https://diataxis.fr/).
• Add Examples section using Sphinx-Gallery (highly WIP), which is referenced in the API reference where functionality is used, similar to Matplotlib, scikit-image etc.
• Restructure API reference into one page per for module, function, class, attribute and method (the latter four have mini-Sphinx-galleries where they are used in the Examples)

Other changes:

• Remove kikuchipy.crystallography from API reference (code will be removed in another PR)
• Remove kikuchipy.simulations.features from API reference (code will be removed in another PR)
• Deprecated kikuchipy.signals.util functions ebsd_metadata() and metadata_nodes() (added to the changelog)

Progress of the PR

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

Minimal example of the bug fix or new feature

See the docs built from this PR.

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.
• New contributors are added to release.py and .all-contributorsrc and the table
  is regenerated.