docs(js): Tanstack Start React quick start update (#17458)

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## 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

## IS YOUR CHANGE URGENT?  

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## SLA

- Teamwork makes the dream work, so please add a reviewer to your PRs.
- Please give the docs team up to 1 week to review your PR unless you've
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Thanks in advance for your help!

## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

## EXTRA RESOURCES

- [Sentry Docs contributor guide](https://docs.sentry.io/contributing/)

Author: inventarSarah