Improve maintianability of navigation.

@jagtalon @moolaza

Inspired once more by the navigation confusion, I whipped up this proof-of-concept for handling navigation more mechanically.  

In brief, I have added an "authoritative" YAML file which lays out the expected navigation of the site.  Then I added a script which uses this YAML to generate ddh-index.md and ddh-prev-next.json

There are a number of assumptions in here some of which may be sub-optimal and/or wrong. :D   This is not intended to be merged straightaway, but to gather some feedback about the concept.

One potentially annoying feature is that if you like the idea of a a human-readable JSON file, "pretty-printed off a perl-structure" isn't the best.. and it will likely generate large diffs for even small changes because of the non-deterministic ordering of hash keys.

Another is that it is predicated on a pretty linear flow, so you can't just branch off into "testing" in the middle of the "goodies" section.

Anyway, feedback appreciated!