docs(signals): remove developer preview flag (#4448)

Author: markostanimirovic

projects/ngrx.io/content/guide/signals/index.md

@@ -1,12 +1,5 @@
 # @ngrx/signals
 
-<div class="alert is-important">
-
-The `@ngrx/signals` package is in <a href="https://angular.dev/reference/releases#developer-preview" target="_blank">developer preview</a>.
-It is ready to try, but may change before becoming stable.
-
-</div>
-
 NgRx Signals is a standalone library that provides a reactive state management solution and a set of utilities for Angular Signals.
 
 ## Key Principles