Use a builder-agnostic TOC reference in user-guide.html

[smashwilson]

Content repositories rendered with the user-guide.html template will fall back to the local TOC if no repository-wide TOC is available. This prevents the TOC from disappearing when the routed template is changed before a content repository's builder is changed to match, as happened in rackerlabs/docs-cloud-load-balancers#134.

@meker12 : This should make it easier to transition content repositories from the deconst-single builder to deconst-serial without losing the TOC in the middle. To gracefully change a repository:

1. Submit and merge a pull request to nexus-control updating config/routes.d/<site>.json to change the template for the content repository from docs-singlepage.html to user-guide.html.
2. Submit and merge a pull request against the content repository changing the builder specified in conf.py from deconst-single to deconst-serial.