docs: add PyPI downloads badge to README

[guillaumegay13]

What this changes

Add a monthly PyPI downloads badge next to the existing npm badges.

Type of change

• Add a model (new YAML file under models/)
• Add a provider (new folder under models/, plus a logo)
• Add or update parameters on an existing model
• Fix incorrect data (default, range, values, applicability)
• Site or tooling (code under src/, docs, CI)

Source

Python package: https://pypi.org/project/modelparams/

Before opening

• npm run validate and npm test pass locally
• Filenames follow the convention: models/<provider>/<model>.yaml, or <model>-subscription.yaml for the subscription route
• No existing parameter was removed (removals are blocked, see CONTRIBUTING)

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