[MINOR] set site outputdir to site

- Add jekyll build option, which is experimental for us.
diff --git a/.asf.yaml b/.asf.yaml
index ef4df19..26f9738 100644
--- a/.asf.yaml
+++ b/.asf.yaml
@@ -56,6 +56,13 @@
   staging:
     profile: ~
     whoami:  asf-staging
-    # top level directory for serving 
-    # index.html
-    # outputdir: site
+    # top level directory for serving index.html
+    # Refer: https://github.com/apache/struts-site
+    # Default directories are 
+    #   content/
+    # or output/
+    outputdir: site
+  
+  jekyll:
+    whoami: master
+    target: asf-staging-jekyll