Note `configure_package_config_file` as an alternative to `configure_file`.

The section was consistent with https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#creating-relocatable-packages, but that CMake documentation does not mention the CMakePackageConfigHelpers, whereas the content here makes explicit reference to that module.

The added "completeness" of this text may not be essential educational content, but it may help readers avoid confusion when encountering the admonitions to use configure_package_config_file instead of configure_file in the CMake documentation at

• https://cmake.org/cmake/help/latest/module/CMakePackageConfigHelpers.html
• https://cmake.org/cmake/help/latest/guide/importing-exporting/index.html#creating-a-package-configuration-file
• https://cmake.org/cmake/help/latest/guide/tutorial/Adding%20Export%20Configuration.html