GH-139: Add config to publish asf-site branch to go subdir (#149)

Fix #139 

The asf-site content will be deployed to https://arrow.apache.org/go/
automatically.

I have created an empty `asf-site` branch and pushed it.

We have to add an index.html there pointing to
`https://pkg.go.dev/github.com/apache/arrow-go/v${VERSION}`.

.asf.yaml

@@ -38,3 +38,6 @@ notifications:
   issues_status: issues@arrow.apache.org
   issues_comment: github@arrow.apache.org
   pullrequests: github@arrow.apache.org
+publish:
+  whoami: asf-site
+  subdir: go