Add documentation on themes, move some themes around

Docs/Themes/postal_codes.md

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+
+
+ Postal codes ( postal_codes) 
+------------------------------
+
+
+
+Postal codes
+
+This theme contains the following layers:
+
+
+
+  - [postal_code_boundary](../Layers/postal_code_boundary.md)
+  - [wrong_postal_code](../Layers/wrong_postal_code.md)
+  - [town_hall](../Layers/town_hall.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:
+
+
+
+  - en
+  - id
+  - hu
+  - de
+  - ca
+  - es
+  - nb_NO
+  - nl
+  - fr
+  - da
+  - pa_PK
+ 
+
+This document is autogenerated from [assets/themes/postal_codes/postal_codes.json](https://github.com/pietervdvn/MapComplete/blob/develop/assets/themes/postal_codes/postal_codes.json)