Improve examples section layout/styling

[doup]

Examples section is now visible on the Learn section, #335. But it's a little bit raw and it could use some styling improvements. Some suggestions by Cart (discord):

I just started a merge for https://github.com/bevyengine/bevy-website/pull/335. If anyone is interested in
sprucing up the example layout a little bit, I think thats worth prioritizing. (ex: rounded corners for the
example canvas to match the code blocks, a nicer back button, ideally some sort of canvas resizing to
make it fit within the bounds of the content areas / work on mobile)

[alice-i-cecile]

#350 is a first step towards this.

[cart]

I'm comfortable closing this now. @doup has done a great job here.