Snap docs/configs/image versions

Related to https://github.com/envoyproxy/envoy/pull/6357. We have two different user bases:
1) People that run master
2) People that run tagged versions

We need to do a better job of having a consistent state that works for people. I think this will involve setting up some templating so that when we build the docs, we snap to a specific SHA for everything, including the image tags. This will need more thought since some of our references to `latest` are checked into code (so maybe some of the Docker files also need to be generated.)