Add automatic creation of tagged release draft

[hakonanes]

Description of the change

• Prepare for a 0.5.1rc1 release.
• Add GitHub Action workflow to perhaps create a tagged release draft if the kikuchipy/release.py file has changed in a commit to or PR merged to the main branch. We can either just publish the draft, or make any changes and then publish. A new version will then be uploaded to PyPI.
• Other minor maintenance changes to the release guide, readme, manifest, etc.

Close #417.

Not sure if a tagged release draft will be made from this PR. If not, I'll make a new one after this is merged.

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