Actually add documentation

Docs/Layers/town_hall.md

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+ town_hall 
+===========
+
+
+
+<img src='https://mapcomplete.osm.be/./assets/themes/postal_codes/townhall.svg' height="100px"> 
+
+
+
+
+## Table of contents
+
+1. [town_hall](#town_hall)
+  - [Basic tags for this layer](#basic-tags-for-this-layer)
+  - [Supported attributes](#supported-attributes)
+
+
+
+
+
+  - This layer will automatically load  [postal_code_boundary](./postal_code_boundary.md)  into the layout as it depends on it:  A calculated tag loads features from this layer (calculatedTag[0] which calculates the value for _postal_code_properties)
+
+
+[Go to the source code](../assets/layers/town_hall/town_hall.json)
+
+
+
+ Basic tags for this layer 
+---------------------------
+
+
+
+Elements must have the all of following tags to be shown on this layer:
+
+
+
+  - <a href='https://wiki.openstreetmap.org/wiki/Key:amenity' target='_blank'>amenity</a>=<a href='https://wiki.openstreetmap.org/wiki/Tag:amenity%3Dtownhall' target='_blank'>townhall</a>|<a href='https://wiki.openstreetmap.org/wiki/Key:building' target='_blank'>building</a>=<a href='https://wiki.openstreetmap.org/wiki/Tag:building%3Dchurch' target='_blank'>church</a>
+
+
+
+
+ Supported attributes 
+----------------------
+
+ 
+
+This document is autogenerated from assets/layers/town_hall/town_hall.json