Update link for docs site setup

Why is this change being made?

We want to encourage Technical Writing team members to get a basic docs build setup working before tackling GDK. We have a Setting Up a Local Environment page in the handbook that talks about this. We also point to the handbook page from our TW onboarding.

When chatting this through with @eread, we noticed that the requirements and dependencies link in the bullet list dropped the reader on the main readme page in the GitLab docs project, instead of the more helpful Setup page. This MR updates the link to the Setup page.

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.
• 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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