Clearly document config.js (base URI and headers)

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.