Related to #4890

The process for upgrading our distros is not always straightforward, and it can be unclear where changes need to be made. We should add documentation to help developers understand where to make the necessary updates when upgrading a distro, while keeping the details high-level in case the process changes in the future.

In particular, the documentation should clarify the differences in the upgrade process for distros used only in stage 1 builds versus those used in both stage 1 and stage 2 builds. The steps involved are not the same, so it’s important to make this distinction clear.