docs: realigned sections for langchain-google-vertexai (#23584)
- **Description:** Re-aligned sections in documentation of Vertex AI
LLMs
- **Issue:** NA
- **Dependencies:** NA
- **Twitter handle:**NA
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Co-authored-by: adityarane@google.com <adityarane@google.com>
Co-authored-by: ccurme <chester.curme@gmail.com>