feat: add CPM.cmake integration support

Add CPM.cmake as an alternative package management option for users
who prefer a zero-install dependency management solution.

Changes:
- Add cmake/examples/CPM_example.cmake: reusable CPM.cmake snippet
- Add cmake/examples/CPM_CMakeLists.txt.example: complete example project
- Update README.md: add CPM.cmake usage documentation

CPM.cmake benefits:
- Zero-install: no need to pre-install package managers
- Cache support: dependencies can be shared across projects
- Simple syntax: easier to use than native FetchContent

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: IronsDu