Use public URL for Technical Writing Fundamentals course link

Please provide a brief explanation for this change:

On /handbook/product/ux/technical-writing/fundamentals/, the current link to the Technical Writing Fundamentals course in LevelUp does not appear to be accessible to users who are not part of the GitLab team. The URL for public access to the course is https://levelup.gitlab.com/courses/gitlab-technical-writing-fundamentals.

This MR updates the URL of the course link to point to the publicly accessible URL on the assumption that GitLab team members will also be able to access the course through that URL.

Please indicate the types of revisions being suggested for the Product Handbook (please check all that apply):

• Small improvement (typos, clarifications, etc.)
• Adding a new section
• Modifying existing section
• Documenting a new process
• Adding a new page or directory
• Other

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• Provided a brief explanation for this change (Say why, not just what)
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  • 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.
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