Added missing.js, a small collection of JS libraries implementing common UI patterns.
Currently contains tabset, menu, and collapsible navbar components.
Breaking: Replaced .sidebar with .sidebar-layout, which works a bit differently.
The source code of missing.css was completely reorganized! The source files
now line up with the pages of the documentation to make it easier to find
code.
Speaking of the docs, they have also received a subtle reorganization.
The development server will now build changes to the CSS way faster.