TR-4558: Update CloudFlare HTTPS setup instructions

[viniciusgiles]

Ticket: https://messagebird.atlassian.net/browse/TR-4558

What Changed

• Updated CloudFlare guide on HTTPS Engagement Tracking, with screenshots of the new UI and more details about correctly forwarding the Host and User-Agent headers to SparkPost.
• Screen recording of the guide:

Screen.Recording.2023-06-29.at.16.12.34.mov

• Link to guide in dev: http://localhost:3000/docs/tech-resources/enabling-https-engagement-tracking-on-sparkpost#step-by-step-guide-with-cloudflare

How To Test or Verify

• Run it locally and make sure the updates are there

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