Fix release version links used by Binder and nbsphinx

[hakonanes]

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

Description of the change

• Fix Binder cannot find 0.3.0 documentation, but CAN find v0.3.0 #300.
• Add an "Unreleased" a "0.3.1" section to the changelog, with this fix mentioned.
• Clarify external download in pattern matching notebook
• Change release version to 0.3.1 in release.py
• Add note about checking that Binder can build the GitHub repository in the release guide

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.