doc: use inkscape instead of dia to generate image files

it makes life a bit easier for CI (less dependencies).

docs/images/Makefile

@@ -7,19 +7,11 @@ BUILD_TARGETS += $(MERKLE_DAG)
 all: $(BUILD_TARGETS)
 
 
-# dia exporters
-
-%.eps: %.dia
-	dia -t eps --export $@ $<
-
 %.svg: %.dia
-	dia -t svg --export $@ $<
-
-# generic converters
-
-%.pdf: %.eps
-	epstopdf $<
+	inkscape -l $@ $<
 
+%.pdf: %.dia
+	inkscape -A $@ $<
 
 clean:
 	-rm -f $(BUILD_TARGETS)