Change or clarify istioctl installation docs

[craigbox]

Right now, we have two pages that tell you how you can get the istioctl binary:

• Download the Istio release, which includes istioctl and samples
• Using the istioctl command line tool, which tells you how to download a tarball which contains only istioctl

@bleggett has pointed out that our "install Istio with istioctl" instructions don't need the samples, and we should only have people download istioctl. However, I don't want do divert people deep to a diagnostic page, so if we are to update these pages, we should move the "download Istioctl only" instructions to the release page, and update the diagnostic page to comment on how

(The alternative is to actively decide that the overhead of the samples isn't a big deal.)