Auto-generate tables of contents for pattern-library pages

[lyzadanger]

Depends on #1053 and #1027

Once we have structured section IDs generated and intra-page fragment linking sorted, it would be helpful to generate automatic tables of contents for documentation pages so one can scan more easily what the page documents. This would be especially useful for pages that document more than one component, as it would be easier to find documentation about the component you're after.