I plan on leaving this open for a bit as additional potential downstream pipeline problems are reported, but this includes optional solutions to address #611 and #613

1. (optional) make Arrow3D a public facing class
2. (optional) remove the pdf build in ReadtheDocs

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.