[Untracked] Add note that Live Activities requires a .p8 key

[hokstuff]

Why are you making this change? (required)

• This escalation by FanDuel when they were using a .pem
• Apple doesn't explicitly say this (even in practice), but we've heard of this behavior from our customers in practice, and our competitors post this as a requirement. We have a Swift SDK follow-up story to validate this behavior

Related PRs, issues, or features (optional)

• Swift SDK tutorials PR

Feature release date (optional)

• N/A

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[vanessaland]

LGTM!

[hokstuff]

@internetisaiah This PR is ready to be reviewed by the Docs team and merged + released afterwards

[hokstuff]

Hi @josh-mccrowell-braze, since Isiah is out, would you be able to take a quick look and then merge + release when you have a chance? Thank you!