docs: catalogue GitHub workflows and manual start options

[lweberru]

Summary

• Add a central GitHub Actions table linked directly from the root README, with all ten current repository workflows and source/Actions links.
• Distinguish automatic PR/push/scheduled triggers, manual Run workflow, reusable callers, inputs, branch/operator gates and environment approvals.
• Explain the Configurator development release, build-only/promotion modes, Plan versus Apply, real cloud effects and model-serving costs. A merge remains separate from deployment authorization.
• Distinguish the two historical Terraform sidebar entries and GitHub-managed Copilot/Dependabot jobs from current workflow files.
• Link contributor/operator guidance to the central catalogue instead of maintaining a second incomplete inventory.

Verification

• Cross-checked workflow definitions against the live GitHub workflow inventory.
• Verified all ten workflow names, source/Actions links and manual-dispatch definitions; 60 local documentation links resolve.
• git diff --check passes.
• Documentation only: no application tests needed; no workflow, repository-setting, deployment or cloud changes.