docs(js): Tanstack Start React quick start update

[inventarSarah]

DESCRIBE YOUR PR

This branch updates the Tantack start React quick start guide to use the new split layout.
I also updated some content in the guide for consistency and clarity -- hope these look okay to you :)

closes: #17456

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Project	Deployment	Actions	Updated (UTC)
sentry-docs	Ready	Preview, Comment	Apr 23, 2026 0:06am

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Project	Deployment	Actions	Updated (UTC)
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