id/name attributes for header elements in the docs

[jeffposnick]

The migration to the new docs serving setup has removed id (or name) tags from the heading elements, taking away the ability to link to specific sections of a larger doc.

E.g. I'd like to link to the "Handlers" section of https://googlechrome.github.io/sw-toolbox/docs/master/tutorial-api.html, but the <h2>Handlers</h2> element doesn't have an id attribute.