Make content have a consistent width

[KTibow]

Current behavior:

• home page uses the nonstandard main-content class, resulting in a width of 1200px that aligns with the header
• docs pages use the standard content class, resulting in a width of 1280px (different from the header's 1200px)

This PR updates all pages to use the content class, and updates the content class to be 1200px wide (through using border-box box sizing).