Add documentation about backporting changes from EE to CE

What does this MR do?

Adds documentation with the workflow to follow when backporting changes from EE to CE, with some special concern for scss changes.

Why was this MR needed?

Onboarding frontend'ers weren't aware of specific process when we do changes in CE files in a EE feature branch.

Suggested via slack to update docs with this.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

N/A