Update subscription management contact workflow following CustomersDot changes

Why is this change being made?

Multiple changes made within this MR:

1. Following several recent CustomersDot workflows introduction, I added headings and reorganized those changes to make them easier to follow.

2. The workflow for adding and changing subscription management contact is now similar, with gitlab-org/customers-gitlab-com#4247 (closed) resolved, introduction of billing account memberships and the ability to update Zuora Sold To contact using CustomersDot. When changing subscription management contact, we should no longer need to invalidate the existing email (by adding _edited) to release it.

3. There were sections that were mentioned numerous times, a section "Other notable workflows involving CustomersDot" is introduced to reference from there instead.

Author Checklist

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