Make the page wider to give more room for README on desktop

[tcbyrd]

Fixes #970

Summary
This is my first PR to this repo, but I saw the Issue above and thought I'd try and tackle it since it's fairly minimal, but is a nice change to help with readability. This is a suggestion to widen the width of the main div to 960px so that the .crate-info .docs div can be increased a bit without having to decrease the size of the .crate-info .authorship div.

Reasoning
I mainly chose 960px since it's a fairly widely accepted standard for content of this type. This allows the documentation to be 640px, which increases the line-length of the pre elements to 65 to help minimize horizontal scrolling.

Before/After

Other Changes
I also changed the responsive screen width option to match the 960px width. The only major side effect here is the authorship div gets a little narrow until it switches to 100% at 900px, but that was already happening at 1040px and it's still readable. The other option is to leave it at 960px and just let the whole page horizontally scroll from 900-960px.

Example of the narrow authorship div @ 915px

Docs Updates
If the team wants to accept this change, for the docs to be consistent, this line in the GitHub pages branch of the Cargo repo will also need to be updated.

I saw there were some discussions about a much bigger redesign, but I thought this may help in the interim since it's a fairly minimal change. I'm open to any feedback! 😄

[carols10cents]

Welcome!! This looks great!! I love the gifs showing the difference too ❤️ ❤️ ❤️ Thank you!!

bors: r+

[carols10cents]

bors: r+

let's try this again

[bors-voyager]

Build succeeded

• continuous-integration/travis-ci/push