Auto merge of #9262 - Matt-Gleich:master, r=Eh2406

🍱 Crop favicon

Hello! At first glance I found doc.rust-lang.org/cargo's favicon to be a bit small compared to other site's favicons. Looked into it and found that the logo hasn't been cropped. All I did was crop the favicon in this PR. As you can see from the image below (new favicon on the right) it looks much better:

![Screen Shot 2021-03-14 at 1 24 38 PM](https://user-images.githubusercontent.com/43759105/111077749-a1ee3280-84c8-11eb-948d-6101ec1a6f77.jpg)

This has been done to crates.io's favicon as well in rust-lang/crates.io#3411. Thanks, you maintainers! :)

Author: bors