Add a CMake option to use system-provided pybind11

[Integral-Tech]

This option enables the use of the pybind11 package available in the official repositories of Linux distributions.

[tencent-adm]

All committers have signed the CLA.

[Copilot]

Pull request overview

This PR adds a CMake option NCNN_SYSTEM_PYBIND11 to enable the use of system-provided pybind11 packages instead of the bundled submodule version, allowing users to leverage pybind11 from their Linux distribution's official repositories.

Changes:

• Added NCNN_SYSTEM_PYBIND11 CMake option in python/CMakeLists.txt
• Implemented conditional logic to use system pybind11 via find_package() when enabled, otherwise fall back to bundled version
• Added documentation in python/README.md explaining how to use the new option

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
python/README.md	Added documentation section explaining the new NCNN_SYSTEM_PYBIND11 option with a usage example
python/CMakeLists.txt	Added option definition and conditional logic to choose between system and bundled pybind11

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[Integral-Tech]

@nihui IMO, the suggestion from Copilot, which uses the REQUIRED keyword in the find_package() call, is not optimal. It would be better to fallback to the submodule if the pybind11 package is not found, similar to how the glslang finding works.

I have just pushed a new commit to include the latest changes :)

[nihui]

Thanks for your contribution !