Update docs.css

Hello,

The max-height attribute isn't affecting the IE8, because IE8 is self closing the header element: `<header class="header"/>`. So the img styling not affect the header image anymore.

I chose `.navbar`, because it's the only content in the header element.

Greetings
Merge pull request #69 from iamsebastian/patch-1

fix(docs): proper header img styling in ie8