content: use HTTPS links where possible, link to download page

Also remove outdated link to the App Engine SDK.

Change-Id: I235359a7ee806d5c537e4654cc8ccc68bea043ec
Reviewed-on: https://go-review.googlesource.com/15059
Reviewed-by: Katrina Owen <katrina.owen@gmail.com>
Reviewed-by: Andrew Gerrand <adg@golang.org>
6 files changed