Unreal Engine: Add notice about loading custom callback handler classes

[tustanivsky]

This PR adds a notice to the Unreal plugin configuration documentation highlighting the importance of plugin loading order when configuring custom callback handlers.

[vercel]

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Project	Deployment	Preview	Comments	Updated (UTC)
sentry-docs	Ready	Preview	Comment	Sep 22, 2025 7:04am

1 Skipped Deployment

Project	Deployment	Preview	Comments	Updated (UTC)
develop-docs	Ignored			Sep 22, 2025 7:04am

[codecov]

Bundle Report

Changes will decrease total bundle size by 15 bytes (-0.0%) ⬇️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
sentry-docs-client-array-push	10.13MB	-6 bytes (-0.0%) ⬇️
sentry-docs-server-cjs	12.85MB	-9 bytes (-0.0%) ⬇️

Affected Assets, Files, and Routes:

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	882.71kB	-0.0%
static/chunks/8321-*.js	-3 bytes	425.87kB	-0.0%
static/bJEGLjVptzTyzYhVRDi2b/_buildManifest.js (New)	684 bytes	684 bytes	100.0% 🚀
static/bJEGLjVptzTyzYhVRDi2b/_ssgManifest.js (New)	77 bytes	77 bytes	100.0% 🚀
static/VArVD2BM6Hujbhodp-*.js (Deleted)	-77 bytes	0 bytes	-100.0% 🗑️
static/VArVD2BM6Hujbhodp-*.js (Deleted)	-684 bytes	0 bytes	-100.0% 🗑️

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

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
1729.js	-3 bytes	1.78MB	-0.0%
../instrumentation.js	-3 bytes	1.1MB	-0.0%
9523.js	-3 bytes	1.08MB	-0.0%