docs/builder.md: remove BuilderByTarget references #75

Job	Run time
paths-filter	3s
Enforce go.mod tidiness	28s
build-test-dev (arm64) / build-test	9m 6s
build-test-dev (amd64) / build-test	12m 25s
build-images-dev (amd64) / build-images	6m 56s
build-images-dev (arm64) / build-images	5m 26s
	34m 24s

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