Use one pass docs image generation (#751)

quicker and cleaner diagram image generation.
Remove etl-graph and mapping-graph targets - redundant

Also, the obsolete "fields" is still in Imposm's code and both names are accepted,
but "fields" is not documented anywhere (PR submitted), and could be removed at any moment.

Our docs were not supporting it until this PR, so renaming it at the same time.

Several images have been updated due to a more inclusive mapping scan
Requires https://github.com/openmaptiles/openmaptiles-tools/pull/147 (merged)

layers/place/mapping.yaml

@@ -20,7 +20,7 @@ tables:
   # etldoc: imposm3 -> osm_continent_point
   continent_point:
     type: point
-    fields:
+    columns:
     - name: osm_id
       type: id
     - name: geometry
@@ -40,7 +40,7 @@ tables:
   # etldoc: imposm3 -> osm_country_point
   country_point:
     type: point
-    fields:
+    columns:
     - name: osm_id
       type: id
     - name: geometry
@@ -70,7 +70,7 @@ tables:
   # etldoc: imposm3 -> osm_island_polygon
   island_polygon:
     type: polygon
-    fields:
+    columns:
     - name: osm_id
       type: id
     - name: geometry
@@ -93,7 +93,7 @@ tables:
   # etldoc: imposm3 -> osm_island_point
   island_point:
     type: point
-    fields:
+    columns:
     - name: osm_id
       type: id
     - name: geometry
@@ -114,7 +114,7 @@ tables:
   # etldoc: imposm3 -> osm_state_point
   state_point:
     type: point
-    fields:
+    columns:
     - name: osm_id
       type: id
     - name: geometry
@@ -144,7 +144,7 @@ tables:
   # etldoc: imposm3 -> osm_city_point
   city_point:
     type: point
-    fields:
+    columns:
     - name: osm_id
       type: id
     - name: geometry