Revise introduction for clarity and conciseness

[Pierrci]

Note

Low Risk
Low risk documentation-only copy edits; main risk is factual/marketing accuracy of updated counts and phrasing.

Overview
Updates the Hub landing page introduction in docs/hub/index.md to be more concise and reposition the Hub as the reference platform for open ML, including refreshed stats for models/datasets/Spaces and explicit mention of private/internal team collaboration.

Removes the now-redundant descriptive paragraph under “What’s the Hugging Face Hub?”, keeping the section focused on the community mission statement.

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[HuggingFaceDocBuilderDev]

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[julien-c]

wynaut 🤷