TW: Update CODEOWNERS

Why is this change being made?

• Fixes ux/technical-writing/ line (misplaced duplication was setting the TW handbook to the Christie rather than to Susan and Craig)

  

• Set Markdown Guide to myself and Mike L, as I think it can be beneficial for the team. I think we could review changes to that page given that I originally authored it and Mike L should be aware of significant changes there since it's also linked from our docs style guide.

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 DRI for the page/s being updated isn’t immediately 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.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

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