docs(nextjs): Add App Router pageload-SSR span linking limitation note (#13839)

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## DESCRIBE YOUR PR
Update docs to reflect trace propagation investigation results for
Next.js versions 14 and earlier with the App Router: Page load to SSR
span linking is not supported.

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## EXTRA RESOURCES

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

---------

Co-authored-by: Lukas Stracke <[email protected]>

Author: 2 people

docs/platforms/javascript/common/tracing/distributed-tracing/index.mdx

@@ -27,6 +27,16 @@ If you run any JavaScript applications in your distributed system, make sure tha
 
 </Alert>
 
+<PlatformSection supported={["javascript.nextjs"]}>
+
+<Alert>
+
+Pageload to SSR tracing is currently not supported on the App Router with Next.js versions up to and including 14.
+
+</Alert>
+
+</PlatformSection>
+
 ## How to Use Distributed Tracing?
 
 <PlatformContent includePath="distributed-tracing/how-to-use/" />