[Guide Wishlist] How to store secrets with AWS

Storing secrets is one of things you want to get *really* right. It'd be great if there was a Buildkite walkthrough for using AWS secret manager for 1 or more projects (with Buildkite).

At the moment, this feels like one of those large surface areas that you just kind of have to know how to manage everything. Seeing as BK doesn't have it's own secrets store, it makes sense to write a guide that covers an often used approach (at least, I think that's what BK does internally?).

Other things this could touch:

* AWS Instance profiles, managed via Elastic stack. (How do we (ideally) give Buildkite access to AWS??)

Thoughts?