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)