docs: add PostHog analytics

[mrlubos]

Closes #1441

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Latest commit: 9420e67

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Name	Status	Preview	Comments	Updated (UTC)
hey-api-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 18, 2024 3:16pm

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commit: 9420e67

[mrlubos]

Ran into an issue with setting up the HTML snippet, filed a ticket with PostHog