Smaller sizes for wordmark

[dherman]

I've added smaller versions of the wordmark image to the assets repo:

• https://github.com/volta-cli/assets/blob/master/wordmark-338x150.png
• https://github.com/volta-cli/assets/blob/master/wordmark-72x32.png

We should update the site to use those in order to optimize page loads.

[dherman]

Hm, annoying. I tried it out and the PNGs look noticeably worse than the JPEG. I'll submit the PR just so we can look at it, but we shouldn't merge it unless we find a way to make it the same quality.

[mikrostew]

I was thinking of converting the Volta logo to SVG. I've done that before with graphics on my own site. It should scale well to the different resolutions used here. I'm planning to try this when I get some time in the next week or two.