docs(external-storage): mark the TypeScript page as Pre-release (#5008)

External Storage is Public Preview overall, but the TypeScript
implementation is not there yet, so the TypeScript page should not
inherit the shared stage.

Use the type prop rather than featureName. getResolvedType checks
FEATURE_RELEASE_TYPES[featureName] before type, so a featureName of
externalStorage would keep winning and the override would be silently
ignored. Dropping featureName lets type apply, which is how the
serverless Worker pages already handle a per-page stage.

Go, Python, and the encyclopedia page keep featureName and stay Public
Preview.

Author: lennessyy

docs/develop/typescript/best-practices/data-handling/external-storage.mdx

@@ -12,7 +12,7 @@ description: Offload large payloads to external storage using the claim check pa
 
 import { ReleaseNoteHeader } from '@site/src/components';
 
-<ReleaseNoteHeader featureName="externalStorage">
+<ReleaseNoteHeader type="prerelease">
   APIs and configuration may change before General Availability. Join the
   [#large-payloads Slack channel](https://temporalio.slack.com/archives/C09VA2DE15Y) to provide feedback or ask for
   help.