Add scalable version of the logo

[The-Compiler]

I needed a scalable version for a (non-public) presentation, and 128x128 is pretty small.

I couldn't find a source for the logo anywhere, so I decided to quickly redraw it based on the original:

and arrived at something which looks pretty close:

I thought I'd add it to the repo in case anyone else needs it too - even though it's not actually used in the docs, next to the .png is the best place to put it, IMHO. If someone had the original logo in a scalable format, that'd be even better, of course.

[webknjaz]

Hey Florian, I recently discovered that the SVG logo is actually present in this repo, just isn't used in sphinx for no reason 🤷‍♂️, and it is located poorly — https://github.com/aio-libs/aiohttp/blob/master/docs/aiohttp-plain.svg.
I've filed an issue to make it actually used (#5677) and somebody even attempted to address it (#5678).

That said, I'm going to close this PR. Thanks for caring!

[The-Compiler]

Ah, turns out I had searched for "logo" in the repo, but not for "svg"... Even better, thanks! 👍