Update the add subscription management contact workflow

Why is this change being made?

Related issue: Document new process for managing Customer asso... (gitlab-org/customers-gitlab-com#5434 - closed)

With the new CustomersDot architecture update for: Iteration 1A: Align CustomersDot BillingAccount... (gitlab-org&8950 - closed) the process to add a manager for a subscription via the CustomersDot Admin has changed.

This is an update to the documentation to align to the new process.

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