Add What's new to TOC (#40079)

In #40077 I missed adding the new file to the TOC and updating the index file.

This PR add those links.

Author: BillWagner

docs/csharp/index.yml

@@ -110,6 +110,8 @@ landingContent:
     linkLists:
       - linkListType: whats-new
         links:
+          - text: "What's new in C# 13"
+            url: whats-new/csharp-13.md
           - text: "What's new in C# 12"
             url: whats-new/csharp-12.md
           - text: "What's new in C# 11"

docs/csharp/toc.yml

@@ -148,6 +148,9 @@ items:
       href: fundamentals/exceptions/how-to-execute-cleanup-code-using-finally.md
 - name: What's new in C#
   items:
+  - name: C# 13
+    displayName: what's New
+    href: whats-new/csharp-13.md
   - name: C# 12
     displayName: what's New
     href: whats-new/csharp-12.md