chore: add organisation docs (#1831)

apps/documentation/pages/users/organisations/billing.mdx

@@ -0,0 +1,19 @@
+---
+title: Billing
+description: Learn how to manage your organisation's billing and subscription.
+---
+
+import Image from 'next/image';
+
+import { Callout, Steps } from 'nextra/components';
+
+### Billing and Subscription Management
+
+Organisations handle billing centrally, making it easier to manage:
+
+- **Unified Billing**: One subscription covers all teams in the organisation
+- **Seat Management**: Add or remove seats across all teams automatically (Teams plan)
+
+You can change plans, view invoices and manage your subscription from the billing page which is accessible from the organisation settings.
+
+![A screenshot of the organisation's billing page](/organisations/organisations-billing.webp)