Document "node data" JSON format

[huddlej]

While chatting with @jameshadfield about a new branch_labels key for node data JSONs produced by augur clades, we realized that we don't currently document the node data JSON format anywhere.

We should start by adding a "node data JSON" section to the data formats of Nextstrain page with a minimal example of a valid node data JSON and a brief description of how these data move through a standard Nextstrain workflow (from raw data to node data JSONs to augur export to auspice).