Fix onboarding: broken links, run scripts, thumbnail

[rcannood]

Describe your changes

Makes the template easier to get started with:

• Fix broken links in README.md and CONTRIBUTING.md: the common_resources instructions moved to TASK_INSTRUCTIONS.md, and the documentation site now uses hyphenated urls (create_task/getting-started/, add-a-method/, add-a-metric/). CONTRIBUTING.md also pointed to a non-existent scripts/run_benchmark/run.sh.
• Add a quickstart to the README to verify the template works before making changes.
• run_test_local.sh now runs as-is on the template's test resources instead of exiting with a TODO; the TODO guards in the other run scripts described steps that didn't match the script contents.
• Add a placeholder thumbnail.svg and reference it from info.image (was placeholder prose), with a step comment like the other placeholders.
• Scope the common test resources to cxg_mouse_pancreas_atlas instead of syncing all of resources_test/common/.

Checklist before requesting a review

• I have performed a self-review of my code

• Check the correct box. Does this PR contain:

  • Breaking changes
  • New functionality
  • Major changes
  • Minor changes
  • Bug fixes

• Proposed changes are described in the CHANGELOG.md

• CI Tests succeed and look good!

[rcannood]

Verified locally with #20 and #22 merged in: viash ns build + run_test_local.sh run green end-to-end, and the scores bracket as expected (random_labels 0.54 < logistic_regression 0.84 < true_labels 1.0).