docs: Add comprehensive Files API and Vector Store integration
documentation
- Add Files API documentation with OpenAI-compatible endpoints
- Create comprehensive guide for OpenAI-compatible file operations
- Reorganize documentation structure: move file operations to files/
directory
- Add vector store provider documentation for Milvus, SQLite-vec, FAISS
- Clean up redundant files and improve navigation
- Update cross-references and eliminate documentation duplication
- Support for release 0.2.14 FileResponse and Vector Store API features
# What does this PR do?
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## Test Plan
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