[GitLab Support] 🔑 Add notes on using mkcert for self-signed certificates

Why is this change being made?

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This MR adjusts the TLS section of the Testing Environment workflow for Support Engineers to:

• add information about using mkcert to generate self-signed certificates
• move the information on self-signed certificates into its own section
• link to the GitLab docs on how to use self-signed certificates in a GitLab instance

The goal of this MR is to indirectly make troubleshooting 🎫 tickets involving self-signed certificates easier. The tools and information for generating and using self-signed certificates in a GitLab instance can be found in this section.

Author and Reviewer Checklist

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