docs: add PyPI version and downloads badges

[guillaumegay13]

What this changes

Keep the PyPI downloads badge, and add a PyPI version badge next to it.

Use shields.io (pypi/dm), which currently reports ~4.1k/month from pypistats. Do not use pepy.tech. pepy includes CI traffic and inflates the count to 14k.

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

• https://pypi.org/project/modelparams/
• https://pypistats.org/packages/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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