TR-4556: Add steps to enable the Host header in support docs

[josecls]

What Changed

• I've updated the steps of the CloudFront guide for "Enabling HTTPS Engagement Tracking on SparkPost". Now users can see how to enable forwarding of the "Host" header properly.
• You can find this guide in http://localhost:3000/docs/tech-resources/enabling-https-engagement-tracking-on-sparkpost#step-by-step-guide-with-aws-cloudfront
• Down below is a screenshot of the changes:
  

How To Test or Verify

• Run it locally and make sure the updates are there

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Content Changes Checklist

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[netlify]

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