Correct URLs of rendered content

[smashwilson]

When a content repository that's mounted at a non-root prefix is rendered, the generated intra-site links aren't going to be correct because they won't include the prefix. They'll need to be rewritten during page presentation to have the correct targets.

[smashwilson]

This isn't manifesting in my developer.rackspace.com prototype, because the docs repo manually appends the /docs prefix to all of its relative links:

Assumptions
------------
This guide assumes the reader has working familiarity with (at the very least):

* `Cloud Servers </docs/cloud-servers/getting-started/>`_
* `Cloud Monitoring </docs/cloud-monitoring/getting-started/>`_

That won't work in the general case because the author of a content repository doesn't "know" where in the final site their content will be mounted. A site administrator should be able to change the base path for any content ID prefix in the control repository and have the rendered content adjust accordingly.

Because we don't need it for the prototype, though, I'll punt this one out the milestone. 👢