Update figures plugin documentation with current behaviors

[cbutcosk]

This PR updates the figures plugin documentation to account for recent refactoring changes to the plugin, and to update links and correct links in the documentation.

[geealbers]

@cbutcosk I made a few small corrections that I noticed in reading it. But otherwise this looks fine to me.

@abrin or @bulbil, do you want to take a quick look and just make sure it includes all the information that you'd expect to find here and doesn't leave you with any questions?

[cbutcosk]

@bulbil Since this documentation is still in review I took the liberty of adding a commit to update it for the data API changes (derivatives and friends) and behavior changes from the recent and pending figures refactor PRs. While these (especially paths) are documented inline in the code it seemed useful to clarify in a more long form way.

I also moved that section on the data APIs higher in the document and lightly reworded some of the intro, both to refine it a bit for developers reading through. @geealbers You may want to take another editorial pass just in case.