Restrict orix to >= 0.7, use cubochoric orientation sampling in user guide

[hakonanes]

Description of the change

• Restrict orix >= v0.7 in setup.py, because cubochoric orientation sampling is used in the user guide, made available in orix 0.7
• Other doc fixes like broken links
• Move doc/changelog.rst to CHANGELOG.rst, and update references to this file elsewhere

Close #440.

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.

[hakonanes]

Will merge after checks pass.