add file name to data-team/platform controlled document entry

Why is this change being made?

Right now all subpages within handbook/business-technology/data-team/platform/ are being treated like controlled documents, requiring approval, whereas only the parent page is listed on the controlled documents list. None of the affected child pages currently have such a designation.

This change references the file for the the /handbook/business-technology/data-team/platform/index.html.md page itself, instead of the directory, so that there can be sub pages which aren't restricted to the same approval process, as they are not controlled documents.

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• 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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