v21.03: update homepage header

The homepage header shows the Nexus 5 with an outdated plasma mobile build. Let's modernize it together with releasing the blog post for the new release.

I've played a bit with it a few months ago, and tried to place three phones on it (n900, pinephone, nexus 5), and each with a different UI. But it was too much... especially for the mobile view of the website. Also we should only use phones that are in main or community, if we use any. And I think only having the pinephone there sends the wrong message, postmarketOS is not only about the pinephone. So I concluded, maybe have no phone at all?

FWIW, I still like the header of the dark theme. I think it's fine to use the nexus 5 there, even though it's not in main or community.

@MartijnBraam: you're doing great design stuff, any idea? (Maybe something based on the wood postmarketOS logo that you made?)

Ideas and mockups welcome!