Docs: Add optional Docker/RunPod quickstart section

[Mars122]

Title: Docs: Add optional Docker/RunPod quickstart section

Summary

This PR documents an optional containerized workflow for DiffSplat users who prefer a pre-wired environment with SSH, JupyterLab, and FileBrowser. It references the docker/runpod/ files added in a companion PR and provides minimal, copy-paste commands for building/running locally or on RunPod.

Motivation

• Many users run on cloud GPU pods or containerized desktops.
• Clear, optional docs reduce environment friction without changing core code.

What’s included

• README: “Optional: Containerized quickstart (Docker/RunPod)” with:
  • Build command: docker build -f docker/runpod/Dockerfile -t diffsplat-runpod .
  • Local run command with --gpus all and exposed ports
  • Service/port explanations (22/80/9090/9999)
  • RunPod notes for HTTP proxy ports and SSH

Non-goals

• No changes to core training/inference code or dependencies.
• The Docker files are proposed separately to keep this PR reviewable on its own.

License

• Docs-only change.