Add GB primary/secondary routes (#1465)

This PR adds an additional pseudo-network for UK non-primary [routes](https://en.wikipedia.org/wiki/Great_Britain_road_numbering_scheme), which are signed with black on white signage with "A" and "B" prefixes. This scheme is described on the [OSM wiki](https://wiki.openstreetmap.org/wiki/Roads_in_the_United_Kingdom). Adding this third psuedo network is needed for generating shields for UK white/black signage for non-primary routes. 
Additionally, this fixes a minor bug under which single-digit refs would have been included.  Single-digit refs don't exist in the UK, because the numbering scheme is always prefixed by a letter, e.g. "A4".

I also included documentation updates and fixed the typo where a view was named "bg" instead of "gb".

layers/transportation_name/transportation_name.yaml

@@ -32,6 +32,7 @@ layer:
       - ca-provincial
       - gb-motorway
       - gb-trunk
+      - gb-primary
       - road (default)
     class:
       description: |