Update labels

Author: MikeRayMSFT

docs/index.yml

@@ -91,7 +91,7 @@ conceptualContent:
         - text: SQL Server Machine Learning Services
           url: machine-learning/index.yml
           itemType: whats-new
-        - text: SQL Server enabled Azure Arc
+        - text: SQL Server enabled Azure Arc (preview)
           url: sql-server/azure-arc/overview.md
           itemType: whats-new
     - title: Install 

docs/sql-server/index.yml

@@ -60,7 +60,7 @@ landingContent:
         url: ../azure-data-studio/what-is.md
       - text: SQL Server on Linux
         url: ../linux/sql-server-linux-overview.md
-      - text: SQL Server for Azure Arc
+      - text: SQL Server for Azure Arc (preview)
         url: azure-arc/overview.md
     - linkListType: quickstart
       links:

docs/toc.yml

@@ -10123,7 +10123,7 @@
     href: /azure/sql-data-warehouse/sql-data-warehouse-overview-what-is  # Would 'index.md' be better?
   - name: SQL Server on an Azure VM >
     href: /azure/virtual-machines/windows/sql/virtual-machines-windows-sql-server-iaas-overview
-- name: Azure Arc enabled SQL Server
+- name: Azure Arc enabled SQL Server (preview)
   items:
     - name: Overview
       href: sql-server/azure-arc/overview.md