[release-branch.go1.8] doc: hide blog content for golang.google.cn

[release-branch.go1.8] doc: hide blog content for golang.google.cn

/blog redirects to blog.golang.org (currently blocked in China)
unless there is a local checkout of golang.org/x/blog, which is
not possible on App Engine Classic.

Change-Id: Ia695e663c9bebcc6c3bedea324c630299eaad4dc
Reviewed-on: https://go-review.googlesource.com/53051
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-on: https://go-review.googlesource.com/81175
Reviewed-by: Russ Cox <rsc@golang.org>