Set up staging website

[bmorelli25]

Summary

Set up a staging environment for Elastic Docs V3 that mirrors our production infrastructure as closely as possible. This environment will allow us to test production-like conditions and validate documentation builds, content, and functionality before they are deployed to the production site. Staging can also be used to validate CSS, HTML, and JS changes before they are merged to prod.

[lcawl]

This also pertains to the versioning requirements (#49) since we've historically not been able to craft whole new content sets spanning multiple "books" and make them available for review unless we made the content publicly visible (i.e. by adding it to the conf.yaml). We would prefer not to be forced to make something part of our public library just for review or preview purposes. In particular, we don't want to be forced to publish unreleased doc versions (like we currently publish "master" docs).