feat: article on looping over objects

[JoshuaKGoldberg]

PR Checklist

• [x] Addresses an existing open issue: fixes #119
• [x] That issue was marked as accepting prs
• [x] Steps in CONTRIBUTING.md were taken

Overview

This is a commonly brought up topic with type narrowing, loops, and objects. A common annoyance. @ladybluenotes brought it up on a stream but this definitely isn't close to the first time it's been mentioned.