Configure docs staging environment

[sean1588]

We currently have a staging version of the site located at www-staging.pulumi.com. We would like to configure this in order to give us a testing ground for future improvements. The staging environment is currently not in use anymore and is quite out of date. We need to update it and figure out a way we can deploy to it to test things out, e.g perhaps a staging branch that deploys to the staging env.