docs(codecatalyst): resolve the GitHub-Actions-path TODO

[joeymussalli]

codecatalyst/README.md offered running the existing GitHub Action inside a CodeCatalyst workflow as a quick alternative, and asked the reader to "verify GitHub-Actions-in-CodeCatalyst support against current AWS docs" -- leaving the one question that decides whether the suggestion is usable unanswered, in the document that makes the suggestion.

Verified. It is supported: the action identifier is aws/github-actions-runner@v1, and the GitHub Action's steps: block is pasted into the CodeCatalyst action's Steps:. Added a worked example.

It is not a drop-in, which is the part worth writing down. AWS states outright that "detailed migration steps are outside the scope of this guide", and their own porting example shows GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }} being deleted, because CodeCatalyst has no GitHub secrets context. Since the token here exists to dodge GitHub's 60 req/hr anonymous limit, someone taking this path and copying the action verbatim would land straight back on rate limiting with no obvious cause. Also noted that anything outside the action's steps: block has to be re-expressed in CodeCatalyst YAML, and that reports and artifacts are configured on the CodeCatalyst action rather than by the GitHub Action.

Kept the recommendation pointing at the native script, and said why.

Documentation only; no code changes.

Refs: https://docs.aws.amazon.com/codecatalyst/latest/userguide/github-action-ref.html