feat: add Python package install commands

[guillaumegay13]

✨ What changed

• Add Python install commands to the homepage and API documentation.
• Link the Python package from the footer, agent guide, and JSON-LD metadata.
• Give each install command its own copy button.

💭 Why

The Python package is published on PyPI but was not discoverable from modelparams.dev.

👤 For users

Python users can copy pip install modelparams directly from the site.

[vercel]

@guillaumegay13 is attempting to deploy a commit to the Manifest Team on Vercel.

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