… access

…CLI for upload

…s install

…in multi_arch_ci

…le workflow

…cess

Addresses review feedback on ROCm#4908:

- multi_arch_ci.yml: gate manifest_diff caller on
  `github.event_name == 'pull_request' || 'push'` so manual dispatch of
  multi_arch_ci no longer fires a no-op manifest-diff job.
- manifest-diff.yml: switch `continue-on-error: true` to
  `${{ github.event_name != 'workflow_dispatch' }}` so CI invocations stay
  non-blocking but direct dispatch surfaces real script regressions red.
- manifest-diff.yml: tag the report step with `id: gen-report` and gate the
  AWS-credentials and upload steps on `steps.gen-report.conclusion == 'success'`
  so a failed report no longer produces a follow-on "no such file" upload
  failure on top of the real error.
- docs/development/manifest_diff.md: rewrite the "How it runs in CI" section
  into Trigger gating / Failure behavior / Where the report lives subsections;
  fix the S3-layout description (TheRockReport.html keeps its name, index.html
  is a separately generated listing page); document fork-PR credential routing
  through configure_aws_artifacts_credentials.