Final fixes for handbook migration script

Why is this change being made?

These should be the final fixes for the handbook migration script before its used to migrate the job-families site to the new handbook. Known issues are:

• Creates duplicate target entries in the redirect.yml
• Markdownlint can't fix all errors
• Doesn't create all required _index.md entries for all job families

This script does the hard part of moving all the content files and preserving their histories but to keep the script pretty generic we stop there.

Author Checklist

• Confirmed that this update should be in the internal handbook rather than public handbook, per the SAFE framework
• 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.
• 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
• 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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