Add artwork to cncf/artwork repo

[craigbox]

Looking at our brand guidelines, please take https://github.com/istio/istio.io/blob/master/static/logos/istio.svg and produce the artwork that is requested for the cncf/artwork repo.

There should be horizontal, and icon images, SVG and PNG, for light and dark background, in the correct brand color.

[craigbox]

@therealmitchconnors has submitted a few of these, but we still need to coordinate horizontal and vertical versions that include the word mark.

(I've raised a Service Desk ticket for the CNCF designers to do this: we have the horizontal version already, and they can make a correctly-spaced version of a stacked/vertical lockup. We should also have them find out why the version we put into the landscape doesn't have the correct color.)

[linsun]

Is this complete?

[craigbox]

The issue was never replied to by the CNCF. I'll chase it.