Link to stream endpoint and mention it in AWS

[cgwalters]

We want people provisioning new systems in an automated fashion
to script it by parsing the stream metadata. Describe that more
including its URL in docs, and also mention it in the AWS section.

[miabbott]

This looks fine to me, but I have a hazy memory that we didn't want to point users at stable.json or the like.

However, looking at the FCOS download page, we do have links to the various JSON files, so my memory must be bad.

[lucab]

This PR does not have any context/reference so it's hard to judge where it's coming from and how was the initial issue framed. I'm fine to merge this as a short-term band-aid, but I have a feeling there is more to uncover behind this.

My guess is that it is the same train of discussion as coreos/coreos-installer#202, whose usecase is to pin references to images (instead of the artifacts themselves).

[bgilbert]

@miabbott We don't want to point people to meta.json or the release metadata, but we very much do want them to use the stream metadata.

@lucab I don't follow the "short-term band aid" part. AFAICT the docs change here is entirely consistent with our updates policy?

[lucab]

@bgilbert that refers to the "here's a JSON, figure it out on your own" paragraph as currently worded. I feel that a longer-term/better UX may involve tweaking coreos-installer or figure out some explicit flow more apt to a "getting started" page.

[cgwalters]

I feel that a longer-term/better UX may involve tweaking coreos-installer

This is also related strongly to coreos/fedora-coreos-tracker#235

[cgwalters]

Since this had some approval, merging but we can always do more followups!