Add placeholder for `datalab-platform` on PyPI

[PierreRaybaut]

To formalize and protect our PyPI namespace under datalab-platform, we should publish a minimal placeholder package to occupy the name.

Tasks:

• Create a new Python package named datalab-platform.
• Include a minimal pyproject.toml with appropriate metadata.
• Add a README.md indicating the purpose of the package and linking to https://datalab-platform.com/.
• Publish the package on PyPI.

This will allow future consistency if we decide to release more packages under the datalab-platform-* naming scheme.