cmake: export an installable pdg package

[zbb2005]

Summary

• install pdg targets and package config files using GNUInstallDirs
• export the pdg::pdg target and restore the public Threads dependency
• add an install-and-consume test that configures a separate project with find_package(pdg 0.1 CONFIG REQUIRED)
• document installed-package usage

Validation

• cmake -S . -B build-contributor -G Ninja -DCMAKE_BUILD_TYPE=Release
• cmake --build build-contributor --parallel
• ctest --test-dir build-contributor -C Release --output-on-failure
• 6/6 tests passed on Windows with MSVC 19.42 and CMake 3.29.5
• verified installed CMake files contain no source-tree or build-tree paths
• Linux and macOS coverage will run through the repository CI matrix

Closes #3