[docs] custom script examples, [build] move CMS make to app via event_handlers.on_build

docs/dev/custom_script_examples/docs.dev.custom_script.validate1.md

@@ -0,0 +1,12 @@
+---
+{
+	"_label": "Restrict Purpose of Stock Entry"
+}
+---
+
+	cur_frm.cscript.custom_validate = function(doc) {
+	    if(user=="user1@example.com" && doc.purpose!="Material Receipt") {
+	        msgprint("You are only allowed Material Receipt");
+	        validated = false;
+	    }
+	}