DEVDOCS-6084 [Update] - Added UB addendum requirement (#760)

<!-- Ticket number or summary of work -->
# [DEVDOCS-6084]


## What changed?
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* Added requirement for signing UB addendum.

## Release notes draft
* Updated Unified Billing documentation to call out the required signing
of the Unified Billing addendum in the partner portal.

## Anything else?
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ping @bc-traciporter @bc-terra @knowakowski 


[DEVDOCS-6084]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6084?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

docs/integrations/apps/unified-billing/index.mdx

@@ -5,6 +5,8 @@ Unified Billing allows merchants to buy apps through the BigCommerce App Marketp
 
 To implement Unified Billing, you need to use the GraphQL API, as well as a [partner portal account](https://partners.bigcommerce.com) for reconciling charges with your other systems.
 
+In order to use Unified Billing, you must sign the Unified Billing addendum within your partner portal account.
+
 <Callout type="info">
   #### Interested in joining the Unified Billing program?
   To indicate your interest in becoming part of this program, please take a few minutes to fill out [this Google form](https://docs.google.com/forms/d/e/1FAIpQLSfupvSoZBeFzJMA5My3Cxbh-DilwsAGNJ4NlMMNYDwuzqMZRA/viewform).