Better document how to configure a CDN for GitLab

We use a CDN for GitLab.com, and it provides a fair amount of value. We should better document so that our customers can benefit, and then consider whether we should add to reference architectures.

Some examples of benefits of a CDN:

• Offloading workhorse and other parts of the GitLab service infrastructure
• Improved performance by keeping assets closer to users
• Earlier use of dynamic image resizing, which does not currently have caching of its own

Proposal

• Introduce a new CDN doc page to https://docs.gitlab.com/ee/install/next_steps.html that describes why might want to use a CDN, link to the omnibus and charts config on how to configure it. And describe how users might grab the assets from a package or image to place in the CDN.
• Update https://docs.gitlab.com/ee/update/zero_downtime.html to have a section on setting up the CDN that points to the new docs.
• Consider whether to include in reference architectures