reorder RenderEngine.cpp to match header; docs and repo hygiene

- RenderEngine.cpp definitions now follow the declaration order in
  RenderEngine.h (public API, factory methods, then private helpers);
  content is otherwise unchanged
- Retire the stale pybind11 comments (the project uses nanobind)
- Include <algorithm> explicitly for std::clamp
- docs/installation.rst: list macOS Intel and Linux aarch64 support
- Add .github/dependabot.yml for monthly GitHub Actions updates