DOC-1868 Document feature UBB biling summary export to CSV (#528)

Author: micheleRP

modules/billing/pages/billing.adoc

@@ -7,6 +7,7 @@ Redpanda Cloud uses various metrics to measure the consumption of resources.
 * All pricing is set in US dollars (USD). 
 * All billing computations are conducted in Coordinated Universal Time (UTC). Billing accrues at hourly intervals. Any usage that is less than an hour is billed for the full hour. 
 * The *Billing* page shows detailed billing activity for each cluster and lets you manage payment methods. Redpanda charges the credit card marked as the default. The most recently added credit card becomes the default payment method, unless you select a different one.
+* To download a CSV summary of your monthly charges per resource, click the download icon on the *Billing* page. You can use the exported file for record keeping or to import into your billing system.
 
 NOTE: Pricing information is available on https://www.redpanda.com/price-estimator[redpanda.com^]. For questions about billing, contact billing@redpanda.com.