Allow articles to be referenced in two categories

[kelshmo]

There are cases where an article should be surfaced in Onboarding as well as another category.

We tested implementation with our current code base by specifying two values in articles header (e.g. category: [onboarding, othercategory]). This was problematic because ordering could not be preserved across categories. We explicitly call out order so the reader is presented information logically.

The way we manage index pages and article headers would need to be altered to accommodate references to a single article in multiple places.