Structure: make it more generic

* changes `orgas` directory to `by_json` (explain what we do: using JSON
    to geneate schema)
* makes Makefile more generic to permit to generate more SVG files in
the future

by_json/Makefile

@@ -0,0 +1,11 @@
+all: orgas.svg
+
+# All .dot files are generated by gen_%.sh file (gen_orgas.sh for example)
+%.dot: gen_%.sh
+	bash "$<" "$@"
+
+%.svg: %.dot
+	dot -Tsvg "$<" -o "$@"
+
+clean:
+	rm -f *.svg *.dot

orgas/Makefile

@@ -1,12 +0,0 @@
-all: organisations.svg
-
-
-graph.dot: gen.sh
-	bash gen.sh graph.dot
-
-
-organisations.svg: graph.dot
-	dot -Tsvg graph.dot -o organisations.svg
-
-clean:
-	rm -f organisations.svg graph.dot