Add documentation for billing contacts

What does this MR do and why?

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

In preparation for [New CDot Nav][Feature flag] Roll out new_cdot_... (customers-gitlab-com#6372 - closed), add documentation for editing a subscription and billing contact.

Screenshots or screen recordings

Subscription contact
Subscription contact dropdown
Billing contact same as subscription contact
Different billing contact
Custom billing contact

How to set up and validate locally

• Navigate to deployed documentation review app (https://main-ee-136525.docs.gitlab-review.app/ee/subscriptions/customers_portal.html#change-your-billing-contact)
• To see the new CustomersPortal UI, navigate to https://customers.staging.gitlab.com/
  • The feature flag is currently default enabled in staging

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.