docs: add account app billing address walkthrough (#430)

## Summary
- add a new Unraid Account walkthrough for updating the billing address
shown on invoice PDFs
- update the instructions to match the current order confirmation and
Manage Purchases -> Orders flow
- keep only the current three screenshots, crop them to the relevant UI,
circle the Update Address target, and redact sensitive values

## Testing
- reviewed the new MDX page content
- verified only the three current walkthrough screenshots remain under
`static/img/unraid-account`
- used targeted OCR checks to confirm redacted personal info and
activation-code text no longer read back from the images

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Added comprehensive guide for updating your billing address in the
Unraid account
* Step-by-step instructions for accessing address updates from order
confirmation or account management
* Details on modifying your billing address through the customer portal
  * Troubleshooting guidance for common scenarios

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: elibosley

docs/unraid-account/update-billing-address-on-file.mdx

@@ -0,0 +1,30 @@
+---
+title: Update Your Billing Address on File
+description: How to update the billing address shown on your invoice using the Unraid account app
+---
+
+## How to update your billing address on file
+
+You can update the billing address that appears on your invoice before downloading the invoice PDF.
+
+1. On the order confirmation page, click **Update Address** in the **Invoice Billing Address** section.
+2. If you already left that page, sign in to your Unraid account, go to **Manage Purchases** -> **Orders**, open the order, and click **Update Address**.
+3. Review the invoice billing address preview.
+4. If the address is correct, click **Download Invoice Now**.
+5. If the address needs to change, click **Update Address** to open the Stripe customer portal, update your billing address, save it, and then download the invoice again.
+
+![Order confirmation page with the Update Address call to action](/img/unraid-account/update-billing-address-new-step-1.png)
+
+![Order details page with the Update Address option](/img/unraid-account/update-billing-address-new-step-2.png)
+
+![Invoice billing address preview before downloading the invoice](/img/unraid-account/update-billing-address-new-step-3.png)
+
+## What happens next?
+
+After you save your changes in Stripe, the invoice PDF you download will use the updated billing address.
+
+## If you need help
+
+- Make sure you're signed in to the same account used for the purchase.
+- If you do not see the order in **Manage Purchases** -> **Orders**, confirm you're using the correct account.
+- Contact support if you still cannot access the order or update the address.