Documentation: Improve structure of Google integrations page (#30992)
This PR restructures the main Google integrations documentation page
(`docs/docs/integrations/providers/google.mdx`) for better clarity and
updates content.
**Key changes:**
* **Separated Sections:** Divided integrations into distinct `Google
Generative AI (Gemini API & AI Studio)`, `Google Cloud`, and `Other
Google Products` sections.
* **Updated Generative AI:** Refreshed the introduction and the `Google
Generative AI` section with current information and quickstart examples
for the Gemini API via `langchain-google-genai`.
* **Reorganized Content:** Moved non-Cloud Platform specific
integrations (e.g., Drive, GMail, Search tools, ScaNN) to the `Other
Google Products` section.
* **Cleaned Up:** Minor improvements to descriptions and code snippets.
This aims to make it easier for users to find the relevant Google
integrations based on whether they are using the Gemini API directly or
Google Cloud services.
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Co-authored-by: Chester Curme <chester.curme@gmail.com>