#51 part 5 — NBN docs merge + livebook rework

* documentation/domains/nbn.md — merged technical content into the
  existing flavor page; dropped Perentie framing; kept the RSP
  spirit-animal table; added topology / inheritance / metrics map
  and scenario walk-through; linked to _nbn_api.md
* documentation/domains/diffo_example_nbn.livemd — rewritten against
  the current code-interface and #49 alias convention; removed the
  broken mine_* / :isAssigned legacy and the JSON:API section;
  added inheritance + metrics demonstrations

Author: matt-beanland