Render/link $ref'ed schemas in the Flow Attributes register

[garethsb]

@joostrovers points out that the Flow Attributes register describes BCP-006-01 profile, level, and sublevel, but doesn't contain the full list of valid values. This is true of several other properties, and we don't necessarily need/want to duplicate the full lists here, especially when we consider adding new sets of enumerated values, like H.264/H.265 profiles and levels.

But the new schemas do include the full list. The top-level flow_video_register.json is rendered and linked from the rendered Flow Attributes register, but the flow_video_base_register.json and flow_video_jxsv_register.json aren't linked.

We could link from specific properties like profile to the relevant schema(s), and/or improve the rendering of the $refed schemas so they are more discoverable. (Latter is mentioned at AMWA-TV/nmos-render-scripts#59)