Usage guidelines for napari logo and screenshots in papers

[jni]

📚 New content request

We've had a few requests for permission to use napari screenshots in papers. We're generally ok with this, so I suggest we make that clear in the docs.

Latest example is:

https://napari.zulipchat.com/#narrow/channel/212875-general/topic/rights.20for.20a.20publication.20with.20a.20screenshot.20of.20napari/near/577710744

As noted in that conversation, the licensing page is probably a good place to add this info or at least link out to it.

See also the text in https://github.com/napari/resources

Cc @TimMonko