Add C++ autodiff example to README and CI workflow

[Copilot]

Summary

Integrates the existing C++ autodiff example (already present in cpp/autodiff/) fully into the repository by updating documentation and CI.

Changes

README.md

• Split the Tools section into Python and C++ subsections, adding the autodiff C++ library to the table.
• Split Setup and Usage into Python and C++ subsections with clear per-language instructions.
• C++ setup covers installing Eigen3 (apt / Homebrew) and copying the header-only autodiff library.

cpp/autodiff/build_and_run_example.sh

• Uses pkg-config --cflags eigen3 to automatically resolve the Eigen3 include path, making the script work cross-platform without hard-coded paths.

.github/workflows/run-examples.yml

• Adds a new run-cpp-example job that:
  1. Installs libeigen3-dev via apt.
  2. Clones the autodiff header-only library at tag v1.1.2 and copies headers to /usr/local/include/.
  3. Builds and runs the example via build_and_run_example.sh.