[x/blog] blog: Mobile-friendly design
[x/blog] blog: Mobile-friendly design

This change complements change golang/go#9062 (because godoc and
the blog share a style sheet) and makes the blog responsive for
mobile.

Change-Id: Ibcd415034c067a6ebc4489dc8b5f32ec9c1a38d6
Reviewed-on: https://go-review.googlesource.com/9063
Reviewed-by: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@golang.org>
X-Blog-Commit: f657641e18cc5c728406b6cf37bea139921a22a1

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