add common-msbuild-item-metadata to toc

Author: ghogen

docs/msbuild/toc.yml

@@ -443,6 +443,8 @@
       href: common-msbuild-project-properties.md
     - name: Common MSBuild project items
       href: common-msbuild-project-items.md
+    - name: Common MSBuild item metadata
+      href: common-msbuild-item-metadata.md
     - name: MSBuild command-line reference
       href: msbuild-command-line-reference.md
     - name: MSBuild .targets files