Virtual backscatter electron imaging RST to notebook

[hakonanes]

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

Description of the change

Virtual backscatter electron imaging RST -> notebook in docs.

Progress of the PR

• Complete conversion (figure out if we should include a GIF, probably yes)
• 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]

Will merge after checks pass.