Add `order` property for articles metadata for sorting

[Marak]

We should also then use this value to sort articles when they are generated for views like the table of contents.

[jfhbrook]

There is already a sort function for the top level of the ToC, but it's pretty ad-hoc at the moment. That said, how would this work? I've seen this sort of arbitrary scale-based ranking system not work as well as one would like, but I'm not sure that there's a reasonable alternative.

[jfhbrook]

This now exists properly. Closing.