Update the fulfillment::purchase handbook page with Issue Board links

Why is this change being made?

Addresses: https://gitlab.com/gitlab-org/fulfillment-meta/-/issues/1057

As per the discussion here: https://gitlab.com/gitlab-org/fulfillment-meta/-/issues/906#note_1180528682, we switched the purchase team milestone boards to be two stable boards instead of creating a new one each time:

1. Active Milestone: https://gitlab.com/groups/gitlab-org/-/boards/5019876
2. Next Milestone: https://gitlab.com/groups/gitlab-org/-/boards/5078830

This MR adds this information to the Purchase team page to make it easier to reference.

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
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • 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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