docs: mention that all official themes are TypeScript-covered

[Josh-Cena]

Motivation

Saw this tweet: https://twitter.com/X8ING_/status/1493180216633528322

Have you read the Contributing Guidelines on pull requests?

Yes

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	53
🟢 Accessibility	100
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	92

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✔️ [V2]

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