vertexai: Add Intro to the Vertex AI library

[SauravP97]

PR Description

Added intro to the Vertex AI library.
Took reference from the Google Gen AI library: https://python.langchain.com/api_reference/google_genai/index.html

Rendered DocString looks like this:

Tagging @lkuligin for review.

Type

📖 Documentation

[lkuligin]

let's start with a short description of all supported integration (and a difference between them, please). So that a user doesn't get an impression that only Llama and Mistarl are supported.
You can find a full list here:

langchain-google/libs/vertexai/langchain_google_vertexai/__init__.py

Line 52 in bd24d50

"ChatVertexAI",

(we don't need to mention things like SafetySettings), but let's mention all relevant Vertex AI integrations, please.

[SauravP97]

Thanks for the review comment, looks like some integrated models were left in my PR. I have added those.

A short description of my recent commit.

1. Added Gemma models integration documentation
2. Added Vision models integration documentation
3. Added a difference between Chat model and Base model for the readers to understand our different flavors of models.