Docs - Improve layout styles for print media

[NiloCK]

Printing documentation from the website results is a big mess. The TOC, Sponsors, and header overlay the main content on every printed page.

I'm unfamiliar with the build process of these docs. It would be hard for me to make these changes and test the result. I suspect that this issue would be fixed by adding something to the effect of

@media print
    .sidebar
        display: none

to the following files:

https://github.com/vuejs/vuejs.org/blob/master/themes/vue/source/css/_sidebar.styl
https://github.com/vuejs/vuejs.org/blob/master/themes/vue/source/css/_header.styl
https://github.com/vuejs/vuejs.org/blob/master/themes/vue/source/css/_sponsors-sidebar.styl

[NiloCK]

Having taken 6 seconds to read the Developing section of this repo's readme, I made the changes myself and have submitted a PR: #1426