docs: Relicense from AGPL 3 to Apache 2

[timmc-edx]

See discussion on https://github.com/openedx/tcril-engineering/issues/289 for background (and authorization). Merge checklist:

• [x] Version bumped
• [x] Changelog record added
• [x] Documentation updated (not only docstrings)
• [x] Commits are squashed