Update Hub API playground link to OpenAPI-based version

[Copilot]

Updates the Hub API playground reference in the documentation to point to the new OpenAPI-based playground and adds information about the OpenAPI specification.

Changes Made

• Updated playground URL from https://huggingface.co/spaces/enzostvs/hub-api-playground to https://huggingface.co/spaces/huggingface/openapi
• Added "OpenAPI-based" description to clarify the nature of the playground
• Added direct reference to the OpenAPI specification file at https://huggingface.co/.well-known/openapi.json
• Updated both the text link and image link to point to the new playground

The changes are minimal and surgical, affecting only the Hub API Playground section in /docs/hub/api.md to ensure users are directed to the correct, up-to-date playground interface.

Fixes #1901.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• huggingface.co
  • Triggering command: curl -I REDACTED (dns block)
  • Triggering command: curl -s -o /dev/null -w %{http_code} REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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[julien-c]

maybe we still keep a link to the Enzo playground for posterity? @enzostvs @gary149