Expand execution docs

[AdamGS]

Summary

This PR extends the execution flow docs, both on the website and in the codebase itself. The general points I've tried to clear up are:

1. How are things called and how all types interact.
2. "Where should feature X go" which has been a pain point in the past
3. Tried to have some docs for more types, even if its just a pointer to some other type.