Logo/header: Unify appearance with wp.org, explore adding the global header

[jasmussen]

Related to WordPress/wporg-mu-plugins#366.

Compared to the main navigation and logo here:

the logo in the /gutenberg subpage is different:

• As a starting point, it would be good to unify those to look the same.

Something to discuss is also whether we should add the Global wp.org header to the /gutenberg section. That could look either like this:

or like this:

[jasmussen]

Seeing an issue here when logged in, and on mobile:

For what it's worth, despite the "show or hide adminbar heuristics" discussed here, I think it would be okay to always hide the adminbar on this page, logged in or not. CC: @StevenDufresne