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

docs/dev/custom_script_examples/docs.dev.custom_script.validate.md

@@ -0,0 +1,12 @@
+---
+{
+	"_label": "Date Validation: Do not allow past dates in a date field"
+}
+---
+
+	cur_frm.cscript.custom_validate = function(doc) {
+	    if (doc.from_date < get_today()) {
+	        msgprint("You can not select past date in From Date");
+	        validated = false;
+	    }
+	}