Add B&SM renew subscription docs

Why is this change being made?

Adding documentation for the Renew subscription feature to the fulfillment guide.

Review app: https://fulfillment-bsm-renewal-docs.about.gitlab-review.app/handbook/product/fulfillment-guide/index.html#billing--subscription-management-features

Related to gitlab-org/customers-gitlab-com#5457 (closed)

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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