Adding Instructions for submitting a Zip Request for Contractors

Why is this change being made?

Changes are being made to the Zip Request a Purchase workflow to clarify the information needed when submitting a request for a Contractor and to ensure the correct approvals are triggered. This MR is to update the Procurement Handbook page to include instructions on how to complete the Zip request and different information/timelines to consider when submitting.

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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