docs: tailor existing-app scenarios by toolchain

[IEvangelist]

Summary

• relabel the Docker Compose comparison as its own scenario
• replace the generic next steps list with toolchain-specific tabs for .NET-centric, TS-centric, Python-centric, and Docker Compose readers
• keep shared follow-up links for executable resources, deployment, and VS Code below the scenario tabs

Validation

• pnpm lint
• pnpm build (succeeded after retrying around a transient Windows file lock in dist)