Add information in relevant upgrade.txt files

Given all of the new navigation code coming in / landed (primary & secondary navigation) we should make heavy mention of how to use it within appropriate upgrade.txt files. As it currently stands the general structure is solidified but minor changes are still happening in the original epic as of MDL creation.

Adrian edit All of this information should be in the developer docs. So in addition to adding comments on the new methods for developers to use, we need to create the documentation that outlines the new classes and how the developers can interact with them with their own plugins.