[docs] added pages from user manual

docs/docs.user.customize.md

@@ -0,0 +1,14 @@
+---
+{
+	"_label": "Customize ERPNext",
+	"_toc": [
+		"docs.user.customize.modules",
+		"docs.user.customize.custom_field",
+		"docs.user.customize.custom_form",
+		"docs.user.customize.print_format"
+	]
+}
+---
+ERPNext offers many tools to customize the system.
+
+You simplify the forms by hiding features you don’t need using Disable Features and Module Setup, add Custom Fields, change form properties, like adding more options to drop-downs or hiding fields using Customize Form View and make your own Print Formats by using HTML Templates. You can also create multiple Letter Heads for your Prints.