Auto merge of #26216 - azerupi:doc-experiments, r=steveklabnik

So I have tried to improve the rustbook engine:

- The sidebar now looks a lot more like gitbook (I thinks it cleaner)
- Added the Open Sans font, in my opinion more readable for prolonged periods of time
- Changed the style for code blocks a little

I encountered 1 problem. In `build.rs` I added this google font url (I commented out the non-relevant parts for clarity) 

```rust
let rustdoc_args: &[String] = &[
    //"".to_string(),
    //preprocessed_path.display().to_string(),
    //format!("-o{}", out_path.display()),
    //format!("--html-before-content={}", prelude.display()),
    //format!("--html-after-content={}", postlude.display()),
    //format!("--markdown-playground-url=http://play.rust-lang.org"),
    //format!("--markdown-css={}", item.path_to_root.join("rust-book.css").display()),
    format!("--markdown-css=http://fonts.googleapis.com/css?family=Open+Sans:400italic,700italic,400,700"),
    //"--markdown-no-toc".to_string(),
];
``` 
As you can see, I had to escape `=` with `=` because the string would get truncated if I didn't. Is that normal behaviour? Is that for security measures? If it is, isn't it a little weak if you can circumvent it by escaped characters? I don't know the reason behind, but I thought it was at least worth mentioning :)

Take your time for this PR, I still want to add multiple improvements:

- Like gitbook, possibility to change font by user
- Put `css` and `js` in their respective files (not hardcoded in rust)
- button to hide sidebar
- ...

So I'm not in a hurry to get this merged ;) But if you think it's good enough to be merged, go ahead. I will make another PR when I have other improvements.

In the image below is a screen of the improvements

![rustbook](https://cloud.githubusercontent.com/assets/7647338/8105345/bf545c74-1038-11e5-962e-b04ebfaf8257.png)

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