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/TODO

@@ -0,0 +1,5 @@
+# TODO list
+
+  * item -> item_types -> patron_types -> circulation policies + organisations
+  * do it in Python
+  * use http://localhost:9200/patrons/_search?size=50 and similar to create all graphs? => problem: you need to launch docker-compose and load data (setup), which takes time