Fix broken links to docs in API plugin repo

Fixes #11.

This fix links to docs in their current locations. I don't know whether the plan is to ultimately migrate all docs from the plugin repo into this docs repo, but figure this makes sense in the short term, either way. 

Sideenote: I noticed that the Github Pages live site at wp-api.org is behind source, in that some live links are broken that are already fixed in this repo. I'm curious what is the process for re-generating the live site, and how often that happens. Can it be automated to recompile when `master` is updated here?