Make CRUD example for C++ in cpp folder

[konard]

Summary

This PR implements the C++ CRUD example for the Doublets library as requested in issue #34:

• ✅ Created cpp/ folder with complete C++ CRUD example implementation
• ✅ Added main.cpp demonstrating Create, Read, Update, Delete operations
• ✅ Added CMakeLists.txt and conanfile.txt for build configuration
• ✅ Added comprehensive README.md with build instructions and documentation
• ✅ Added GitHub Actions workflow for C++ builds with GCC and Clang

Implementation Details

The C++ example follows the same pattern as the C# and Rust versions:

1. Creates a doublet link using UnitedMemoryLinks
2. Updates the link to reference itself (creates a point: 1→1)
3. Reads and displays all links in the store
4. Cleans up by resetting and deleting the link

Test Plan

• Code compiles with C++20
• CMakeLists.txt properly configured with Platform.Data.Doublets dependencies
• README.md matches the style and structure of other language examples
• GitHub Actions workflow added for automated building

🤖 Generated with Claude Code

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Resolves #34