Clearly document config.js (base URI and headers)

[handrews]

Equivalent of cloudflare/doca#5

baseUri is used for the base when resolving href for examples. With draft-04 it is the only way to do so. draft-06+ have base, so this isn't really needed

globalHrefSchema is more or less what it says, corresponding to the draft-07 headerSchema. Need to explain its behavior (it's overridden by headerSchema, not allOf'd) and perhaps direct people to more flexible uses of headerSchema even with older drafts.