Add a tutorial page working through the hello example step by step

If you pull together a tutorial flow - this would be great within it. It's a validation that the image was pushed and is available, but otherwise doesn't really align with what this plugin does, so it's maybe a touch awkward without all the other context.

_Originally posted by @heckj in https://github.com/apple/swift-container-plugin/pull/99#discussion_r2047286392_

The issue names no file, test, or documentation entry point. First locate the existing hello example and surrounding tutorial structure, then trace the image-push validation in context. Done means a step-by-step tutorial page explains the hello flow and makes that validation understandable within the plugin workflow.

Written by the indexing model from the issue text.