Add documentation on themes, move some themes around

Docs/Themes/grb.md

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+
+
+ GRB import helper ( grb) 
+--------------------------
+
+
+
+This theme is an attempt to help automating the GRB import.
+
+This theme contains the following layers:
+
+
+
+  - [named_streets](../Layers/named_streets.md)
+  - [type_node](../Layers/type_node.md)
+  - [osm-buildings](../Layers/osm-buildings.md)
+  - [service_ways](../Layers/service_ways.md)
+  - [generic_osm_object](../Layers/generic_osm_object.md)
+  - [address](../Layers/address.md)
+  - [crab_address](../Layers/crab_address.md)
+  - [grb](../Layers/grb.md)
+  - [current_view](../Layers/current_view.md)
+  - [gps_location](../Layers/gps_location.md)
+  - [gps_location_history](../Layers/gps_location_history.md)
+  - [home_location](../Layers/home_location.md)
+  - [gps_track](../Layers/gps_track.md)
+
+
+Available languages:
+
+
+
+  - nl
+ 
+
+This document is autogenerated from [assets/themes/grb/grb.json](https://github.com/pietervdvn/MapComplete/blob/develop/assets/themes/grb/grb.json)