docs(js): Update react-router docs for import flags

[chargome]

Documentation for React Router was updated to provide an alternative setup for environments where the NODE_OPTIONS --import flag is not supported.

• A new subsection, "Alternative Setup for Hosting Platforms," was added under the "Update Scripts" section in docs/platforms/javascript/guides/react-router/index.mdx.
• This section addresses platforms like Vercel and Netlify, where direct import of the instrumentation file is necessary.
• Instructions were added to import ./instrument.server.mjs at the top of entry.server.tsx.
• A code example demonstrates this direct import.
• A warning alert was included to clarify that this alternative method results in incomplete auto-instrumentation and should only be used when the --import flag is not an option.

ref getsentry/sentry-javascript#15198

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
sentry-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 23, 2025 0:12am

1 Skipped Deployment

Name	Status	Preview	Comments	Updated (UTC)
develop-docs	⬜️ Ignored (Inspect)	Visit Preview		Jun 23, 2025 0:12am

[codecov]

Bundle Report

Bundle size has no change ✅

Affected Assets, Files, and Routes:

view changes for bundle: sentry-docs-client-array-push

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
server/middleware-*.js	-5.55kB	1.0kB	-84.74%
server/middleware-*.js	5.55kB	6.55kB	555.3% ⚠️
static/ycK2_TFBBx-*.js (New)	684 bytes	684 bytes	100.0% 🚀
static/ycK2_TFBBx-*.js (New)	77 bytes	77 bytes	100.0% 🚀
static/QfEJKV3U7ZGS6xl7ELTOP/_buildManifest.js (Deleted)	-684 bytes	0 bytes	-100.0% 🗑️
static/QfEJKV3U7ZGS6xl7ELTOP/_ssgManifest.js (Deleted)	-77 bytes	0 bytes	-100.0% 🗑️