GPL documentation

[scott-martens]

Before publicizing GPL in Finetuner, we need:

• Documentation on how to do it
• Maybe a notebook with a minimum example?

see also: https://github.com/jina-ai/finetuner/issues/736

[LMMilliken]

For the release this week, just a notebook will be added explaining how to do it, a more complete explanation will be included as part of the docs refactoring (#736)