docs: explain how to update billing info and payment method (#934)

Add a "Updating Billing Information and Payment Method" section to the
  Plans & Billing page covering the path from Settings > Plans & Billing >
  Manage Plan into the Stripe billing portal, where users can change their
  payment method and use "Update information" to edit company name, billing
  address, billing email, and tax information.

  Note that this is NetBird Cloud specific functionality — self-hosted
  deployments are not billed through the Stripe billing portal.

src/pages/manage/settings/plans-and-billing.mdx

@@ -96,6 +96,29 @@ Your subscription cycle starts on the first day of your original subscription. I
 </Note>
 
 
+## Updating Billing Information and Payment Method
+
+<Note>
+This is a NetBird Cloud feature. Self-hosted deployments are not billed through the Stripe billing portal.
+</Note>
+
+To update your payment method or billing details, navigate to `Settings` > `Plans & Billing` and click `Manage Plan`.
+
+<p>
+    <img src="/docs-static/img/manage/settings/plans-and-billing/manage-plan.png" alt="manage-plan" className="imagewrapper-big"/>
+</p>
+
+You will be redirected to the Stripe billing portal, where you can:
+
+- Add or replace a payment method, and set which one is used by default.
+- Click `Update information` to edit your company name, billing address, billing email, and tax information (such as your VAT ID).
+- Review your invoice history and download past invoices.
+
+<Note>
+Changes to your billing details apply to invoices issued after the update. Existing invoices are not modified retroactively.
+</Note>
+
+
 ## Deleting an Account
 
 If you [delete your account](/manage/settings/delete-account), the final charge for your usage will be processed during the next daily statistics update cycle at 2 AM UTC.