Update datetime field to correctly store in UTC

[mackeyguenther]

fixes #1400

1. update datetime field (UI component & validation) to serialize/deserialize the value in UTC time, and render the value in the user's local timezone
2. update datetime documentation to make it clear that datetime stores its value in UTC time, and displays it in the user's local timezone in the editing interface

[changeset-bot]

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Latest commit: 5b3a070

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[larssont]

This would be really nice to see 👍
Is there any update on this PR?

[kylejrp]

@BarnabyBishop @emmatown @JedWatson Happy holidays, hope you all are doing well. 🎄 Would love to see this reviewed and get merged into the next release!

This is causing headaches for getting the local timezone right. If I write the date 2025-12-28 22:20 (10:20PM) in the Keystatic UI while in pacific time, it gets written in the frontmatter as 2025-12-28T22:20:00.000Z which then incorrectly gets rendered by my site as December 28th 2025, 2:20:00pm which is off by an entire 8 hours. 😞