Document platform support and Docker Desktop limitations

[dionhaefner]

Mosaic works best on Linux with Docker Engine. On macOS and Windows (Docker Desktop), users hit several issues that aren't well documented:

• ARM image mismatches on Apple Silicon (solver images are built for x86_64)
• --gpus flag failures when NVIDIA Container Toolkit isn't available
• VM overhead from Docker Desktop's Linux VM layer

Goal: Add a "Platform support" section to docs/getting-started.qmd that:

1. States the recommended setup (Linux + Docker Engine)
2. Documents known Docker Desktop failure modes and workarounds (WSL 2, Rosetta emulation)
3. Adds a troubleshooting entry for the --gpus / "no known GPU vendor" error

The README already has a brief platform note — this issue is about expanding the docs with actionable detail.