Fix the broken WFS demo layer, improve GeoServer WFS documentation

[slead]

The demo WFS layer 'world countries' is failing:

{
  "id": "countries",
  "name": "World countries (WFS)",
  "type": "WFS",
  "url": "https://demo.boundlessgeo.com/geoserver/opengeo/wfs",
  "typeName": "opengeo:countries",

as this layer is no longer available. Replace this with a working example, and improve the documentation on how to connect to a GeoServer WFS / GeoJSON layer.