genai: docs added complete and structured README for langchain-google-genai package

[SYED-M-HUSSAIN]

PR Title

docs: add complete and structured README for langchain-google-genai integration

PR Description

This PR adds a complete and updated README.md for the langchain-google-genai package. It includes:

• Overview and installation instructions
• Environment setup for API keys
• Chat model usage (gemini-pro, gemini-pro-vision, Gemini 2.0 Flash)
• Support for multimodal inputs and outputs
• Embedding generation using embedding-001
• Retrieval-Augmented Generation (RAG) using GoogleVectorStore
• Code examples and best practices

The goal is to make the package easier to use and contribute to by providing clear, structured, and complete documentation.

Relevant issues

N/A

Type

📖 Documentation

Changes (optional)

• Added full README.md with structured sections
• Clarified usage examples and model names
• Improved consistency and formatting for Markdown

Testing (optional)

Manually verified code snippets with local environment using Gemini API keys. No functional or logic changes made, documentation only.

Note (optional)

This PR affects only documentation. All functional behavior remains unchanged.

[SYED-M-HUSSAIN]

@lkuligin Could you please review this PR, thanks.