Add SVG logo

[wbnns]

We should use an SVG logo instead of a PNG to improve quality and responsiveness across devices:

[l33d4n]

I checked the PR list but didn’t find any mentions to tag here, closed issue, right?

https://github.com/bitcoin-dot-org/developer.bitcoin.org/blob/master/_theme/static/sphinxbootstrap4.css_t#L395

.logo-light {
    content: '';
    width: 270px;
    height: 33px;
    background: url('./Logo-light.svg') no-repeat;
    background-size: contain;
}