Collapsible menu styles. Full width page layout. Slightly improved styles.

This may be a little opinionated, but I do think it's an overall improvement.

1. The menu is now collapsible. This becomes very important once your API becomes more than just a couple of resources, which is typically the case. It's much easier to jump around between resources and quicker to find the resources you need.
1. Changed it to be a full page width layout. I often found the menu would wrap over the content if the resources had slightly long names. It also feels like it's better use of screen real-estate.
1. Slightly improved styles. The menu now uses colours to coordinate with their respective `METHOD` colours. Also chose more fitting icons and improved some font sizes and some hover effects.

Let me know what you think.