docs/PUBLIC_ECR.md is referenced but was never committed

`scaled-execution-containers/README.md:92`, right after the Option A / Option B split, says:

> **See [docs/PUBLIC_ECR.md](docs/PUBLIC_ECR.md) for detailed comparison and setup.**

That file was never committed. `docs/` contains only `SECURITY.md`, `SNS_NOTIFICATIONS.md` and `TROUBLESHOOTING.md`.

This is the only place the Option A vs Option B comparison is promised. Line 153 mentions the `USE_PUBLIC_ECR=true` setting but not the trade-offs, so someone deciding between the prebuilt public image and building their own has nothing to go on.

I left this one out of #79 (the broken-links PR) on purpose - the other 15 links were pointing at files that exist under a different name, so those were path fixes. This one needs content or a decision to drop the reference, which is yours to make rather than mine.

Two ways to close it, whichever you prefer:

- write `docs/PUBLIC_ECR.md`, or
- drop the sentence and fold a short comparison into the Deployment Options section

Happy to send a PR for the second if that is the direction you want.

Same class of thing, if it is useful to track together: `aws-managed-definitions/nodejs/version-upgrade.md` refers to three files under `document_references/` (`npm-check-updates-command.md`, `common-migration-patterns.md`, `known-debug-patterns.md`) and that directory does not exist anywhere in the repo. The Graviton definition does ship a real `document_references/`, so the nodejs one reads like it was written against a directory that never landed.

Start with scaled-execution-containers/README.md around lines 92 and 153, then inspect the existing docs/ files and the repository’s public ECR deployment references. Confirm the intended treatment with the maintainer before changing anything. Done means the Option A versus Option B guidance is available at a valid location and the README no longer points to missing documentation.

Written by the indexing model from the issue text.