Add amenity=drinking_water (#765)

* Add amenity=drinking_water

Fixes #569.

From test:
Generated 118,970 tiles in 0:06:20.7, 312.4 tiles/s +1.3%, 155.5 bytes/tile +0.0%

* updated devdoc image  - Co-authored-by: Yuri Astrakhan <yuriastrakhan@gmail.com>

layers/poi/mapping.yaml

@@ -24,6 +24,7 @@ def_poi_mapping_amenity: &poi_mapping_amenity
   - courthouse
   - dentist
   - doctors
+  - drinking_water
   - embassy
   - fast_food
   - ferry_terminal