Move code snippets into docs content and version them

Previously the code content was only kept in master and was
Subsequently referenced by the versioned doc contents. This causes
A few problems.. so we will version it along with the doc content.

To get this to work we make a handful of changes:

* Move the docs up into just opa/docs/content/*, we’re going to remove
  The difference between local/dev and production builds soon.
* Move the “code” directory into the content dir, it is content tied
  To the markdown files already there.
* Change the Hugo config to ignore the code directories
* Change the “code” shortcode to load the versioned code snippet
* Change load-docs to copy the code directory in addition to the other
  markdown content.

Signed-off-by: Patrick East <east.patrick@gmail.com>