Create filesretrieval integration page

[zyantw]

Adding integration guide for the FilesRetrieval tool

Staged: https://deploy-preview-2166--adk-docs-preview.netlify.app/integrations/filesretrieval/
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Agent's PR: #1534
Original Isuee: #1521

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PR Documentation Claim	Technical Verification	Code Citation
Constructor Parameters: Requires name, description, input_dir and accepts an optional embedding_model typed as BaseEmbedding.	Verified. The __init__ signature in files_retrieval.py perfectly matches this requirement, enforcing keyword arguments (*).	FilesRetrieval.__init__
Default Model: Defaults to gemini-embedding-2-preview via GoogleGenAIEmbedding with a batch size of 1.	Verified. If embedding_model is None, it calls a helper function that initializes exactly this model configuration.	_get_default_embedding_model
Dependencies: Requires llama-index-embeddings-google-genai for the default implementation.	Verified. The default model loader wraps the import in a try...except block, explicitly raising an ImportError directing users to install this package.	ImportError Handling
LlamaIndex Architecture: Uses VectorStoreIndex to read local directories and perform search.	Verified. The tool utilizes LlamaIndex's SimpleDirectoryReader to load the input_dir and builds a retriever using VectorStoreIndex.from_documents().	Retriever Initialization

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