Limit "additional resources" section to non-core information

Why is this change being made?

Removes core TW information from the the TW set up page, and replaces with some initial entries to handy non-core resources:

• Before: https://about.gitlab.com/handbook/engineering/ux/technical-writing/setup/
• After: https://eread-refactor-additional-tw-resources.about.gitlab-review.app/handbook/engineering/ux/technical-writing/setup/index.html

The idea would be that onboarding steps should be limited to the onboarding issue, and this page becomes a handy non-mandatory list of resources.

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this change to the correct DRI(s)
  • 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 in 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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