This PR is solving https://github.com/openmaptiles/openmaptiles/issues/406, partially also https://github.com/openmaptiles/openmaptiles/issues/320.
It is changing source of data for zoomlevel 6 - 8 in `waterway` layer to OSM (relation `waterway = river`), this way it corresponds with `water` layer.
Testing on Europe, z0 - z9 tiles increase size from 20.7 MB to 21.4 MB. We can discuss the level of generalization, but I think this size change is not critical.
During the import-osm step, there are three generalized tables for the waterway that are not used for subsequent processing.
Thanks @lazaa32 for the notice.
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)