cmake/docs: fix macOS install RPATH and update install notes

[JulianGrg]

This PR improves macOS installation support in two ways.

First, it updates the CMake install configuration so that the installed dw executable receives a macOS runtime library search path. Without this, dw can fail after installation with:

Library not loaded: @rpath/libkdtree.dylib
Reason: no LC_RPATH's found

The CMake change adds an Apple-specific INSTALL_RPATH pointing from the installed executable directory to the installed library directory:

@loader_path/../${CMAKE_INSTALL_LIBDIR}

This should allow the installed dw binary to find libkdtree.dylib without requiring users to patch the binary manually with install_name_tool.

Second, this PR updates the macOS installation documentation with:

• Homebrew dependency installation instructions, including libomp
• OpenMP setup notes for macOS
• A fallback CMake invocation for cases where omp.h is not found
• Troubleshooting instructions for the previous libkdtree.dylib runtime-path issue

The documentation keeps the manual install_name_tool workaround as a troubleshooting fallback, but the CMake change should avoid the need for it in normal installations.

I kept the old macOS install notes, but they can be removed if wished.