feat: devnet docker module and howto

[miller-da]

DevNet Module

Quickstart has been limited to LocalNet deployments since June 2025. Prior to this PR, there was no easy way to teach Canton Network developers how to deploy quickstart to DevNet. Developers wanting to move from LocalNet to DevNet needed to research their own solutions for authentication and onboarding differences.

DevNet Docker Module

I created the docker/modules/devnet/ module to address this gap. This module provides:

1. Complete working configuration for deploying quickstart to an external splice-node validator
2. Pre-configured services that handle the LocalNet-to-DevNet differences automatically
3. Reference implementation that developers can study, modify, and extend

Why a Separate Module

I chose to maintain DevNet as a separate directory rather than adding flags to LocalNet for several reasons:

1. Clean separation of concerns: LocalNet and DevNet have fundamentally different architectures. LocalNet runs everything, DevNet connects to external infrastructure. Mixing them would create confusing conditional logic.
2. Clear mental model: When a developer looks at docker/modules/devnet/, they see what's needed for DevNet deployment.
3. Independent evolution: LocalNet and DevNet can evolve independently. Changes to LocalNet for local testing scenarios won't accidentally affect DevNet configurations.
4. Discoverability: The existence of the DevNet directory signals that DevNet deployment is supported and documented.

Extending the Quickstart Promise

This module extends the original promise of Quickstart to streamline Canton Network application development.

[miller-da]

Pairs with docs-website add devnet deploy to toc
https://github.com/DACH-NY/docs-website/pull/1494