Revert "remove dotnet docfx global flag"

This reverts commit b6307fb359.

build/azure-pipelines.yml

@@ -215,7 +215,7 @@ stages:
             inputs:
               targetType: inline
               script: |
-                choco install docfx --version=2.59.4 -y
+                dotnet tool install -g docfx
                 if ($lastexitcode -ne 0){
                     throw ("Error installing DocFX")
                 }