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

docs/dev/custom_script_examples/docs.dev.custom_script.validate3.md

@@ -0,0 +1,17 @@
+---
+{
+	"_label": "Restrict Cancel Rights based on Certain Order Value"
+}
+---
+Add a handler to `custom_before_cancel` event:
+
+	cur_frm.cscript.custom_before_cancel = function(doc) {
+	    if (user_roles.indexOf("Accounts User")!=-1 && user_roles.indexOf("Accounts Manager")==-1
+	            && user_roles.indexOf("System Manager")==-1) {
+	        if (flt(doc.grand_total) > 10000) {
+	            msgprint("You can not cancel this transaction, because grand total \
+	                is greater than 10000");
+	            validated = false;
+	        }
+	    }
+	}