Changes to new Hiring Approach static site - adding sourcing fact sheets

Why is this change being made?

As part of this issue, we're going to add sourcing fact sheets to the handbook, nest them under Sourcing, and link from the static jobs page. This MR adds the sourcing fact sheets page to the handbook

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the change relates to any part of the project other than updates to content and/or data files (e.g. team.yml) please make sure to ping @gl-static-site-editor in a comment for a review and merge.
  • If the DRI for the page/s being updated isn't immediate clear, then assign it to your manager.
  • 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

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Closes #7384