[8.16](backport #4730) Remove list of supported generative AI providers

[mergify]

Summary

The connector docs are to be the source of truth for supported Generative AI providers. #4143 intended to remove a supported providers list from the documentation. Unfortunately, during the cherrypick process (#4407), conflicts prevented this deletion from making it to main. This PR syncs our AI assistant docs with the changes proposed, discussed, and merged in #4143.

A more recent and related comment supporting this decision is in #4410 (comment).

• Closes [AI Assistant]: Document recommendations for using the Gemini connector #4410
• Closes [Request]: Uncomplete requirements section in Observability AI Assistant doc #4610

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  This is an automatic backport of pull request Remove list of supported generative AI providers #4730 done by Mergify.

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