[waiting] Generate content ID references instead of relative URLs.

[smashwilson]

Implements the Sphinx preparer part of deconst/deconst-docs#105.

Remaining Work

• Generate {{ to(target-content-id) }} instead of relative URLs.
• Generate just the target content ID for next and previous links in the metadata envelope.

[smashwilson]

This will modify the preparer to render nunjucks-ish markup for all Sphinx-generated internal links, and use a contentID element instead of a url element for next and previous navigation:

{
  "toc": "...",
  "layout_key": "default",
  "previous": {
    "contentID": "https://github.com/rackerlabs/docs-core-infra-user-guide/cloud-config/compute/cloud-servers-product-concepts/server-events",
    "title": "Monitoring events on your cloud server"
  },
  "next": {
    "contentID": "https://github.com/rackerlabs/docs-core-infra-user-guide/cloud-config/compute/cloud-images-product-concepts",
    "title": "Understanding Cloud Images"
  },
  "body": "... <li><a class=\"reference internal\" href=\"{{ to('https://github.com/rackerlabs/docs-core-infra-user-guide/cloud-interfaces/gui/cloudservers-gui') }}#cloudservers-gui\"><em>Cloud Servers and the Cloud Control Panel</em></a>...",
  "title": "Actions for Cloud Servers"
}

Notice that URL fragments will still be intact.

🚧 This PR is ready to go, but I can't merge until all of deconst/deconst-docs#105 is ready to 🚢. 🚧