content/static: update header items to explicitly set margins
content/static: update header items to explicitly set margins

li items are given a top margin on some pages (release notes being
a notable example). Explicitly set all margin values to ensure
no top margin is added.

Change-Id: I4fec8130409b916cd3b10b53bfc8863c8235d28f
Reviewed-on: https://go-review.googlesource.com/c/website/+/180759
Reviewed-by: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website