Create Headshot Pro case study

[joethreepwood]

Changes

Closes https://github.com/PostHog/posthog.com/issues/8682 Closes https://github.com/PostHog/posthog.com/issues/8684 Closes https://github.com/PostHog/posthog.com/issues/8336

• [x] Approved by user
• [x] Add testimonial to new product page

DO NOT MERGE

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Name	Status	Preview	Updated (UTC)
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[corywatilo]

Merged into #8875. Changes should be made in there going forward.