Fix GitHub Models docs links

[DouweM]

#2114 was green but docs failed on main: https://github.com/pydantic/pydantic-ai/actions/runs/16081790577/job/45387524408

[hyperlint-ai]

PR Change Summary

Updated documentation to include GitHub Models and fixed links in the API providers section.

• Added GitHubProvider to the API providers documentation
• Included GitHub Models link in the models index

Modified Files

• docs/api/providers.md
• docs/models/index.md

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