fix(CI): less output from docfx

Author: richardschneider

appveyor.yml

@@ -60,7 +60,7 @@ after_build:
 # See https://github.com/dotnet/docfx/issues/1752#issuecomment-308909959
   - set MSBUILD_EXE_PATH=C:\Program Files\dotnet\sdk\2.0.0\MSBuild.dll
   - copy doc\MSBuild.dll.config "C:\Program Files\dotnet\sdk\2.0.0\MSBuild.dll.config" /Y
-  - docfx doc\docfx.json
+  - docfx doc\docfx.json --logLevel Warning
   - 7z a -tzip docs.zip doc\_site
   - appveyor PushArtifact docs.zip
   - if defined git_token gh-pages -d doc\_site -m "new docs %GitVersion_FullSemVer%"