Add note about setuptools branch

[nicholasjng]

Similar to the already existing note on the Bazel branch.

I pushed this to make my findings on setuptools-powered nanobind extension builds available to interested users. This is work in progress, but builds already on my machine, and follows the custom builds user guide in nb_combined.cpp on Linux.

---

Contains my research on a pybind11 migration project I'm doing on the side. If there is interest on your part, I can also write a small explanatory guide on setuptools for the nanobind docs. I'm aware that CMake is the preferred build system, but I figured that removing hurdles for getting users of other packaging solutions on board can be worth it.

[wjakob]

I think that setuptools is something from which many people are trying to actively move away. Adding all the details needed to compile nanobind code in every project (instead of abstracting them away into a separate package) makes this somewhat messy, and the dynamic nature of the setup.py causes many issues. So I think I prefer not advertising this.

[nicholasjng]

Makes sense, thanks!