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

docs/dev/custom_script_examples/docs.dev.custom_script.fetch.md

@@ -0,0 +1,20 @@
+---
+{
+	"_label": "Custom Script: Fetch Values from Master"
+}
+---
+To pull a value of a link on selection, use the `add_fetch` method.
+
+	add_fetch(link_fieldname, source_fieldname, target_fieldname)
+
+### Example
+
+You create Custom Field **VAT ID** (`vat_id`) in **Customer** and **Sales Invoice** and want to make sure this value gets updated every time you select a Customer in a Sales Invoice.
+
+Then in the Sales Invoice Custom Script, add this line:
+
+	cur_frm.add_fetch('customer','vat_id','vat_id')
+	
+
+---
+See: [How to create a custom script](!docs.dev.custom_script.html)