Improve Ergonomics around manipulating DocNode data in processors

[grahamboree]

The current approach to manipulating data in a ConfigProcessor is a bit clunky. Specifically, you need to first convert the existing DocNode to a ComposedDocNode, which isn't super clear. Then, the methods for manipulating ComposedDocNodes are pretty limited.

This feels like it needs revisiting with an eye on more complex ConfigProcessors.