Add metadata, MVA notebooks, update README with Sphinx gallery

[hakonanes]

Signed-off-by: Håkon Wiik Ånes hwaanes@gmail.com

Description of the change

• Migrate user guide reST documentation files explaining the signal metadata structures and multivariate analysis (MVA) to Jupyter Notebooks. The MVA one includes part of the denoising procedure explained in this conference paper
• Add a thumbnail gallery from the notebooks in the README, via nbsphinx. This adds a [doc] dependency on sphinx-gallery >= 0.6.
• Minor additions and changes to the contributing guide

Close #238.

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

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

[hakonanes]

Yeah... The gallery won't work in the README file. Will have to find a better way to include it there.

[hakonanes]

Visit https://kikuchipy--285.org.readthedocs.build/en/285/ built from this PR to see the thumbnail gallery in action!