New_cdot_navigation docs integration

What does this MR do and why?

Related to customers-gitlab-com#7008 (closed)

Cannot be merged until customers-gitlab-com#6372 (closed) is complete (new_cdot_navigation feature flag rollout).

An integration branch of updates for the new_cdot_navigation feature.

Updates:

Documentation link	Original MR	Status
payment methods documentation	Update CustomersPortal payment docs (!136124 - merged)	Reviewed
account owner/company details documentation, GL account linking documentation	Update personal and company information docs (!136126 - merged)	Reviewed
GitLab SaaS/SM & QSR payment method pages	Update GitLab SaaS/SM payment docs (!136520 - merged)	Reviewed
Billing/subscription contact (New section)	Add documentation for billing contacts (!136525 - merged)	Reviewed
Contact troubleshooting (New section)	Billing contact troubleshooting (!137401 - merged)	Reviewed

to support the new CDot Navigation UI flow

Screenshots or screen recordings

See original MRs for examples of new UI flows

How to set up and validate locally

• Run the review-docs-deploy pipeline job
• Click on the deployed app to view changes

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

doc/subscriptions/customers_portal.md

@@ -73,6 +73,48 @@ To change your company details, including company name and VAT number:
 1. Edit the company details.
 1. Select **Save changes**.
 
+## Change your subscription contact
+
+The subscription contact is the primary contact for your billing account. They receive subscription event notifications and information about applying subscription.
+
+To change the subscription contact:
+
+1. Sign in to the [Customers Portal](https://customers.gitlab.com/customers/sign_in).
+1. On the left sidebar, select **Billing account settings**.
+1. Scroll to the **Company information** section, then to **Subscription contact**.
+1. To select a different subscription contact, select from the **Billing account manager** dropdown list.
+1. Edit the contact details.
+1. Select **Save changes**.
+
+## Change your billing contact
+
+The billing contact receives all invoices and subscription event notifications.
+
+To change the billing contact:
+
+1. Sign in to the [Customers Portal](https://customers.gitlab.com/customers/sign_in).
+1. On the left sidebar, select **Billing account settings**.
+1. Scroll to the **Company information** section, then to **Billing contact**.
+
+   - To change your billing contact to your subscription contact:
+
+     1. Select **Billing contact is the same as subscription contact**.
+     1. Select **Save changes**.
+
+   - To change your billing contact to a different billing account manager:
+
+     1. Clear the **Billing contact is the same as subscription contact** checkbox.
+     1. Select a different billing account manager from the **User** dropdown list.
+     1. Edit the contact details.
+     1. Select **Save changes**.
+
+   - To change your billing contact to a custom contact:
+
+     1. Clear the **Billing contact is the same as subscription contact** checkbox.
+     1. Select **Enter a custom contact** from the **User** dropdown list.
+     1. Enter the contact details.
+     1. Select **Save changes**.
+
 ## Change your payment method
 
 Purchases in the Customers Portal require a credit card on record as a payment method. You can add