Officially support SmolLM3 using the Transformers backend

[hmellor]

Adds support for SmolLM3 by:

• Adding it to the supported models docs
• Adding an entry in the model registry
• Adding an entry in the test model registry

[github-actions]

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[gemini-code-assist]

Code Review

This pull request adds official support for the SmolLM3 model using the Transformers backend. The changes include updating the documentation to list SmolLM3 as a supported model, adding it to the model registry to enable official support via the generic Transformers wrapper, and including it in the test registry. The changes are straightforward and appear correct. I have no major concerns.