Added create plan

Author: clairekinde11

src/content/docs/integrate/webhooks/add-manage-webhooks.mdx

@@ -108,6 +108,7 @@ customer.invoice_overdue;
 customer.meter_usage_updated;
 customer.payment_failed;
 customer.payment_succeeded;
+customer.plan_created;
 customer.plan_assigned;
 customer.plan_changed;
 ```