Migrate docs to Vitepress

[drwpow]

Changes

Uses Vitepress for documentation. Why?

• Better maintainability. Less custom code, more template-y solutions (that are updatable and patchable)
• Better a11y. Provides great a11y out-of-the-box, including [custom containers]
  (https://vitepress.dev/guide/markdown#custom-containers) that replace bad blockquote abuse
• Better readability. Clearer anchor links, searching, and better mobile styles
• Easier contributions. No custom docs code; now it uses a community standard with great support
• Better styling. It just looks better than the old custom docs code.

Docs-only change; no code changes.

How to Review

(Note: the Cloudflare preview will fail at first because this needs to merge before that is switched over)

Checklist

N/A

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Latest commit: e46214c

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