fix(crawlability): Wrote nginx rules to render partials for base routes.

Crawlers were making requests to docs.angularjs.org/?_escaped_fragment_, as well as to other base routes such as /api/ /guide/ /cookbook/ /tutorial/. These were 404ing because the blanket rewrite rule was looking for partials at the path /partials/.html instead of /partials/api/index.html, etc. The problem was solved by creating an nginx map to associate base URIs with the correct partial path.
fix(_escaped_fragment_): Updated _escaped_fragment_ urls to contain a <base> tag