Threat Insights FCL - Identify and update documentation on how to test different environment and user licenses locally

This issue is part of our Threat Insights Frontend Feature Change Lock.

Identify and update documentation on how to test different environment and user licenses locally

Key results

1. Determine what additional steps we should take to verify changes locally.
2. Update documentation

Emulate SaaS mode locally:

1. export GITLAB_SIMULATE_SAAS=1
2. gdk restart
3. Checked "Allow use of licensed EE features" (need to confirm location of this option)

MR thread reference - !86639 (comment 965243425)