fix: broken and duplicated Google documentation

[felixarntz]

Background

A recent backport must have messed this up: For some reason or v6 documentation for Google is missing the new "Managed Agents" section and a few other changes - and equally worse, the sidebar shows both "Google Generative AI" (v6 naming) and "Google" (v7 naming), the latter of which is a 404 (and duplication anyway).

Summary

• Port the additions that were in 15-google.mdx but missing from 15-google-generative-ai.mdx (after verifying all of these actually apply to v6 code too)
• Fix the mentioning of AI SDK repo internal examples/ai-functions files, which had previously been fixed in 15-google.mdx but was still in 15-google-generative-ai.mdx
• Delete the duplicate 15-google.mdx file

Checklist

• All commits are signed (PRs with unsigned commits cannot be merged)
• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)