docs: update Google Generative AI docs

[patrickloeber]

Hi, Patrick here from DeepMind. I wanted to update the Google Generative AI docs to use newer models and simplify some stuff. This PR includes the following changes:

• Update some links
• Add a simple generate text example
• Use gemini-2.5-flash as default (same as on the Gemini API docs)
• Clarify thinking budgets and document includeThoughts
• Remove dynamic retrieval (this is only supported with the old 1.5 and considered as legacy)
• Update model name in image generation snippet
• Mention support for Streaming
• Simplify Search example and merge with "Sources" section
• Add info about implicit and explicit caching (this is copied over from the v4 docs and looks newer)
• Copy Gemma section from v4 docs and move the section to the bottom
• Remove info about tuned models as this is no longer actively supported in the Gemini API

[lgrammel]

The docs on main are for v5. You seem to have reverted some AI SDK 5 code to AI SDK 4 in the examples (e.g. provider options). Can you check that this works with AI SDK 5?

[patrickloeber]

@lgrammel thanks for taking a look, and good catch! The caching example used the v4 code, I uploaded a fix

[lgrammel]

please fix the prettier issues