Support for table of contents (TOC)

[simurai]

Originally posted in primer/css#816 by @gladwearefriends

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Bring back the bullet point outline list of links at the top of each guide

For example, on the old Colors guide

Glancing at this list lets me confirm if I am on the correct page or not (i.e. some topics as our border radius utility arent intuitive to me as to which page they live on - border? box?). The list also lets me jump to relevant section on the page quicker instead of scrolling.

Perhaps it was taken out for good reason but I just wanted to share my two cents!