Add documentation for Self-Managed Trial Guidelines

Why is this change being made?

The SaaS Trial Guidelines was recently added to handbook. This guideline is meant to help users on Self-Managed get started with a trial and provide them with their options.

Edit: Everything below the "Populating your group with project data" section is copied from the SaaS Trial guidelines, as well parts recommending the separation between production data and trial data.

This guide helps to differentiate the different trial options between Self-Managed and SaaS. For example, it is currently not possible to do a trial in your SaaS namespace if it has a subscription, however it is possible in Self-Managed.

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, and the content is SAFE
• 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
  • The when to get approval handbook section explains the workflow in more detail
• 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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