Create an explanation doc on what is a `datapackage.json` file and what is inside it

[lwjohnst86]

Wondering if this guide should include a quick intro to what the datapackage.json file is before talking about it here? Could also be a more overall (but still quite brief) introduction of what a Sprout data package contains?
There’s a bit of intro on L28-30, but maybe it would be nice to include at the beginning of the guide to “set the scene”?

Originally posted by @signekb in #1109 (comment)