Add a running GDK as a requirement for frontend fixtures

What does this MR do and why?

GitLab Frontend documentation explains how you should fetch frontend fixtures but it does not mention that running a GDK instance before running any of these commands is a requirement. This MR intends to fix that by adding a note that you should have a running GDK instance before running any of these commands.

Screenshots or screen recordings

How to set up and validate locally

To validate that it fails without GDK:

1. Stop all the services that GDK runs at the startup if you have GDK running already
2. Execute bin/rake frontend:fixtures inside gitlab folder

The command should fail with error messages that look like that:

An error occurred while loading ./spec/frontend/fixtures/u2f.rb.
Failure/Error: require_relative '../config/environment'

FrozenError:
  can't modify frozen Array

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.