installing `pybind11` as a build time python package dependency

[shubhbapna]

When we try to build docling-parse from source, it tries to clone the pybind11 repository and then set the includes path. However, we are running this build in a network isolated environment which causes the clone step to fail.

An alternate to this would be to install pybind11 as a build time python package dependency and use its API in build.py to set it up:

• https://pybind11.readthedocs.io/en/stable/compiling.html#modules-with-setuptools
• https://github.com/octavifs/poetry-pybind11-integration

[tiran]

I made a similar change to Triton a while ago, https://github.com/triton-lang/triton/pull/4450