release: improve documentation and clarity around pure_docker upgrades

Created by: coury-clark

We have a discrete step and repository in the release to update a pure docker mode of deployment. This release step is very convoluted and error prone, and requires a lot of "tribal knowledge" to perform.

1. Update the link to the documentation to go directly to the pure docker release mode instead of linking to the overall page
2. Write a section that clarifies the purpose of the rolling customer branch
3. Update the instructions to make it clear when merging branches that some files contents are different on the customer branch than on master, and clarify that those contents should remain after the merge
4. Possibly investigate some kind of improvements here such that this merge isn't so hacky