doc: hide content blocked by GFW when served in CN
    
    Change-Id: Ia3d0fa0517d96f0672d3c5ce7e7b7bb8bd0ce093
    Reviewed-on: https://go-review.googlesource.com/60070
    Reviewed-by: Chris Broadfoot <cbro@golang.org>
    (cherry picked from commit 98603e7af6056734e0f5bee5f05b3623ca5189de)
    Reviewed-on: https://go-review.googlesource.com/88735
    Reviewed-by: Andrew Bonventre <andybons@golang.org>