gddo-server/assets: update banner

The banner for godoc.org is updated to this design:

https://photos.app.goo.gl/sm8w3QFF4ewoJUNu9

Change-Id: Iaf7e12218ace5d1f87c6d0131b86f71d4835406b
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/278152
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Trust: Julie Qiu <julie@golang.org>