handbook: first pass at defining the word "licence"

Created by: LawnGnome

This came out of a discussion in the Badger Batcher sync today: we use the word "licence" in multiple contexts, and it's confusing how they interact in practice when discussing feature availability and licensing.

Some of this is ground I'm pretty comfortable with (FOSS licensing), but some of this treads on ground that I'm less comfortable with (anything that makes it sound like I'm a lawyer), so I'm going to take a slow, methodical approach to getting this merged, rather than mashing the merge button as soon as someone approves it.

First up, I'm opening this in draft so @malomarrec can review it and make sure it (a) makes vague sense, (b) aligns with what we discussed this morning, and (c) should actually live in the product section of the handbook. After that, I'll start bothering other people who I won't tag just yet to ensure that we're not telling authoritative sounding lies here.

:badger: