Add bevy_bird SVG for simpleicons.org (#3672)

# Objective

Add an icon to [simpleicons.org](https://simpleicons.org/), so that we can use it with [shields.io](https://shields.io/) to make some nice shields/badges.

## Solution

Create a bevy bird SVG according to this guideline https://github.com/simple-icons/simple-icons/blob/develop/CONTRIBUTING.md#icon-guidelines, meaning i made [bevy_icon_dark.svg](https://github.com/bevyengine/bevy-website/blob/master/static/assets/bevy_icon_dark.svg) monochromatic and ran through all the steps in the icon-guideline. The result looks like this:

![grafik](https://user-images.githubusercontent.com/11967542/149509545-0748af23-ff52-42e6-a5bc-c1c7ce43e1fe.png)

We still need to open a PR in the https://github.com/simple-icons/simple-icons repo.

Author: Weasy666