feat: add llamaindex integrations

[Lovegoodstudy]

Purpose of the change

Add a sample integration with LlamaIndex.

Description

Add a sample integration with LlamaIndex.

examples/llamaindex/mem_machine_memory.py The MemMachineMemory class, which implements the BaseMemory base class, is provided for integration.
examples/llamaindex/example.py Provide an example of integration with LlamaIndex SimpleChatEngine.
examples/llamaindex/guide.md A guide how to integrate MemMachine with LlamaIndex.
examples/llamaindex/test_mem_machine_memory.py Test for MemMachineMemory class

Fixes/Closes

Fixes #(issue number)

Type of change

• New feature (non-breaking change which adds functionality)
• Documentation update

How Has This Been Tested?

Run test_mem_machine_memory.py for unit test.
Manually verify by running example.py according to guide.md.

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• Manual verification (list step-by-step instructions)

Test Results: [Attach logs, screenshots, or relevant output]

Checklist

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• I have signed the commit(s) within this pull request
• My code follows the style guidelines of this project (See STYLE_GUIDE.md)
• I have performed a self-review of my own code
• I have commented my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

Maintainer Checklist

• Confirmed all checks passed
• Contributor has signed the commit(s)
• Reviewed the code
• Run, Tested, and Verified the change(s) work as expected

Screenshots/Gifs

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Further comments

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