Modernize sync-repo.sh script and migrate to scriptexamples

[calm329]

What

Modernizes the sync-repo.sh script by migrating from deprecated gsutil to gcloud CLI and relocates it to the helm-www repository.

Why

The script in helm/helm was outdated:

• Used deprecated gsutil instead of the recommended gcloud CLI
• Hadn't been updated since Helm v2
• Served no internal purpose in the helm/helm repository
• Was only referenced by documentation in this repository

How

• Created new scriptexamples/ directory (following the existing sdkexamples/ pattern)
• Added modernized sync-repo.sh using gcloud storage rsync commands
• Updated docs/howto/chart_repository_sync_example.md:
  • Changed prerequisites from gsutil to gcloud CLI
  • Updated script reference to new /scriptexamples/sync-repo.sh location
  • Replaced all gsutil rsync commands with gcloud storage rsync equivalents
  • Updated documentation links to current Google Cloud docs

Fixes helm/helm#31727

[calm329]

@scottrigby Could you please review my PR?