Make side bars collapsible.

[ReinisSprogis]

Hi. I mostly use web browser on my second vertical monitor when reading documentation. But sidebars take a lot of screen space and usually dartpad is very squished. So I often end up deleting side bars in page source. Would be nice if there would be an option to hide them. dartdoc-sidebar-left and dartdoc-sidebar-right. Here is example how it looks like with and without side bars.