BENCH-9859: Add a minimal Artifact Registry container example

[samhornstein]

Adds a minimal three-step example: create an Artifact Registry repository, build and upload the supplied container with Cloud Build, and run hello-nextflow with that image on Workbench. The commands read project and region from wb workspace describe and reuse the image URI and resolved scratch bucket. No local Docker installation is required.

The artifact_registry profile applies upper_container only to convertToUpper; the workflow fails before task submission if the image parameter is missing. The main README links to the short example, with advanced registry setup left to the Workbench documentation.

Dependency

Stacked on PR #26, which introduces per-process container declarations. Merge #26 first, then retarget this PR to main if needed.

Validation

• Previous workflow checks confirmed that only convertToUpper changes image and that a missing image fails before task submission.
• Shell syntax checks passed for all commands in both changed documents. Project and region extraction matched the supplied workspace output; relative links and whitespace checks passed.
• Cloud Build submission and the Workbench workflow run still need verification on Workbench.

Jira: BENCH-9859