gddo-server/assets: update banner

The banner for godoc.org is updated based on designs from b/172852539.

Screenshot:
https://photos.app.goo.gl/FZB7VBo8ZKjQqhhh9

Change-Id: Idd7709ee292843f59528d5c7a380bbaaba06bb84
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/274698
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>