Use cases documentation.

## Edit existing content 📝

### Pages to modify
- /usage/*

### Required changes

After #4209, #4221, #4222, and #4223, `Scenarios` will be the only thing left under `Usage`. Rename `Usage` to `Use cases`, flatten the `Scenarios` subheading, and write documentation that provides **Explanation** of different `Use cases` for Infection Monkey. The pages currently under `Scenarios`, as well as some of the **Explanation** on the home page should provide a good starting point.