[Documentation] Add comments explaining currentList.js and redux files

The currentList.js file is what maintains the state for the visible list of courses that is eventually used by the CourseList component to render a list of CourseCards. It would behoove us to understand this file a little better. Currently, it doesn’t have any comments explaining what it does so consider adding some to explain the core functions.

There are also a handful of redux files that add layers of abstraction that may be confusing to new developers on the project.

Things to document for redux:

Selectors and learning how to differentiate them, for example, what is the difference between appSelectors, simpleSelectors, and courseCard selectors.

What this app.js file is doing may also be helpful

Explain the requests slice and how there are multiple slices for the store

In terms of where these comments should exist, it'd probably be the most helpful to either include them in the files themselves and/or the README.

Start with src/data/redux/app/selectors/currentList.js and compare its selectors with appSelectors.js, simpleSelectors.js, and courseCard.js. Read src/data/redux/hooks/app.js and the src/data/redux/requests slice, then document the selector distinctions, app hook, and store slices in the relevant files or README. Done means a new developer can understand these abstractions and how CourseList uses the visible course list.