[Docs Rewrite] Create "Treat Reducers as State Machines" page

What docs page is being added or updated?

This bit: https://redux.js.org/style-guide/style-guide/#treat-reducers-as-state-machines

• Section: #treat-reducers-as-state-machines
• Page: Redux Style Guide

For Updating Existing Content

What updates should be made to the page?

Add detailed explanation for "Treat Reducers as State Machines" section

Do these updates change any of the assumptions or target audience? If so, how do they change?

No, but it would make it easier to understand with a code example provided.