docs: Clarify JAVA_HOME setup and initial build expectations for all OSes (#37415)

Author: Esmelealem

CONTRIBUTING.md

@@ -41,6 +41,11 @@ These steps and instructions on getting started are outlined below as well.
 - A [GitHub](https://github.com/) account.
 - A Linux, macOS, or Microsoft Windows development environment.
 - Java JDK 11 (preferred, or 8, 17, 21) installed.
+  - General note:
+    - Set `JAVA_HOME` to the JDK **installation directory**, not the `bin` directory
+    - (Window for example: `C:\Program Files\Eclipse Adoptium\jdk-11.x.x`)
+    - If multiple JDK versions are installed, ensure Java 11 appears first on `PATH`
+    - The initial Gradle build may take 10–15 minutes due to dependency downloads
 - Latest [Go](https://golang.org) 1.x installed.
 - [Docker](https://www.docker.com/) installed for some tasks including building worker containers and testing changes to this website locally.
 - For SDK Development: