Fix orphaned links for Sandbox Cloud templates, clarify Terraform Environment instructions, Sandbox Cloud Realm

Why is this change being made?

While attempting to create a test instance on GCP, I ran into some trouble following the instructions. My questions were answered in this Slack thread, which inspired this MR.

It was determined that if I have these questions, other users probably do, too. The intention here is to clear up any confusion in the steps, and to make the instructions more accessible for those who may be unfamiliar with Terraform or GCP.

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.
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• 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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