Use cases documentation.

[mssalvatore]

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.