Add JS v9 migration guide

[lforst]

DESCRIBE YOUR PR

Adds the migration guide for v9 of the JS SDK.

Resolves getsentry/sentry-javascript#14290

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[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
sentry-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 6, 2025 10:49am

2 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
changelog	⬜️ Ignored (Inspect)	Visit Preview		Feb 6, 2025 10:49am
develop-docs	⬜️ Ignored (Inspect)	Visit Preview		Feb 6, 2025 10:49am

[codecov]

Bundle Report

Changes will increase total bundle size by 195 bytes (0.0%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
sentry-docs-server-cjs	10.7MB	201 bytes (0.0%) ⬆️
sentry-docs-client-array-push	9.38MB	-6 bytes (-0.0%) ⬇️

Affected Assets, Files, and Routes:

view changes for bundle: sentry-docs-server-cjs

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
1729.js	-3 bytes	1.63MB	-0.0%
../instrumentation.js	-3 bytes	965.23kB	-0.0%
9523.js	-3 bytes	941.45kB	-0.0%
../app/[[...path]]/page.js.nft.json	70 bytes	376.07kB	0.02%
../app/platform-redirect/page.js.nft.json	70 bytes	375.99kB	0.02%
../app/sitemap.xml/route.js.nft.json	70 bytes	373.96kB	0.02%

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

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
static/chunks/pages/_app-*.js	-3 bytes	866.04kB	-0.0%
static/chunks/9557-*.js	-3 bytes	391.11kB	-0.0%
static/EFUHGhGjy2CoJJhT9U_FZ/_buildManifest.js (New)	578 bytes	578 bytes	100.0% 🚀
static/EFUHGhGjy2CoJJhT9U_FZ/_ssgManifest.js (New)	77 bytes	77 bytes	100.0% 🚀
static/jE8pbJgL59FVTCAsYSVAJ/_buildManifest.js (Deleted)	-578 bytes	0 bytes	-100.0% 🗑️
static/jE8pbJgL59FVTCAsYSVAJ/_ssgManifest.js (Deleted)	-77 bytes	0 bytes	-100.0% 🗑️

[chargome]

Looks good!

[coolguyzone]

Thanks for this big update 💪 I added a few suggestions, looks good!