docs(js): Update React Router Framework quick start guide to use split layout

[inventarSarah]

DESCRIBE YOUR PR

This branch contains:

• split layout update in the React Router Framework quick start guide (manual setup)
• small adjustments to the existing split layout quick start guide for React router framework (wizard setup)

I didn't apply the split layout to some of the Expandables because it made the readability worse.

Closes: #17613

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Project	Deployment	Actions	Updated (UTC)
sentry-docs	Ready	Preview, Comment	May 5, 2026 11:56am

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Project	Deployment	Actions	Updated (UTC)
develop-docs	Ignored		May 5, 2026 11:56am