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.

assets/branding/bevy_bird_simpleicons.svg

@@ -0,0 +1 @@
+<svg role="img" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><title>Bevy Engine</title><path d="M20.687 11.262h-.006c-.125.39-.221.814-.239 1.297-.035.905.057 1.457-.047 2.044-.104.587-.413 1.164-1.197 2.07-.933 1.079-2.413 1.913-4.122 2.195a8.847 8.847 0 0 1-1.006.555c1.33.501 2.721.528 3.823.166 1.943-.637 1.793-1.095 2.663-2.429.274-.42.591-.741.909-1.005.906.395 1.798.649 2.354.586.299-.693-.008-1.496-.516-2.256.417-1.296-.216-2.594-1.619-3.053a3.007 3.007 0 0 0-.997-.17Zm1.052 1.904a.524.524 0 0 1 .407.177.536.536 0 0 1-.056.754.537.537 0 0 1-.755-.041.536.536 0 0 1 .056-.754.54.54 0 0 1 .348-.136ZM13.71 3.201c-.76-.004-1.539.299-2.208.948-.878.854-1.151 1.622-.997 2.58l.042.263-.263-.042c-1.931-.313-4.32-.155-7.04.945.8.342 2.267.914 3.895 1.696l.516.248-.561.114c-2.15.436-4.701 1.694-7.03 4.441 1.042-.081 3.172-.332 5.393-.328 1.236.002 2.452.087 3.444.344.991.257 1.783.691 2.081 1.435l.02.051-.01.054c-.048.277-.096.527-.141.732-.104.47-.51.849-1.042 1.187-.533.339-1.208.634-1.909.885-1.403.503-2.896.824-3.556.885-.14.013-.195.064-.25.161a1.001 1.001 0 0 0-.088.414c-.005.215.023.395.047.516 4.291.352 7.481-.718 9.784-1.632 2.587-1.186 4.223-3.529 4.578-5.638.222-1.321.174-1.969-.04-2.521-.215-.551-.626-1.041-1.148-1.977a6.108 6.108 0 0 1-.638-1.74l-.024-.115.092-.074c.959-.769 1.739-1.608 2.023-2.219-.582-.618-1.586-.749-2.711-.629l-.092.01-.066-.066c-.598-.611-1.342-.923-2.101-.928Zm.336 1.428a.715.715 0 0 1 .7.55.712.712 0 0 1-.515.86.712.712 0 0 1-.851-.53.713.713 0 0 1 .515-.861.694.694 0 0 1 .151-.019Zm4.717 1.673a3.122 3.122 0 0 0-.76.08l-.051.014c-.289.285-.612.572-.957.854.111.49.276 1.002.568 1.525.507.909.925 1.394 1.17 2.025.245.631.288 1.372.061 2.724-.282 1.675-1.303 3.449-2.919 4.759 1.255-.366 2.325-1.042 3.033-1.86.764-.883 1.019-1.382 1.108-1.886.09-.503.006-1.055.042-1.991a5.343 5.343 0 0 1 .315-1.595l.039-.111.117-.016c1.046-.136 1.992-.403 2.48-.718-.157-.706-.824-1.253-1.707-1.67l-.084-.039-.021-.091c-.28-1.203-1.219-1.979-2.434-2.004Zm.608 1.486c.062 0 .124.009.184.028.32.103.498.456.394.782h-.001a.621.621 0 0 1-.774.408.62.62 0 0 1-.393-.781.623.623 0 0 1 .59-.437Z"/></svg>