[clang][docs] Clarify CMake version requirement for CMAKE_LINKER_TYPE

[Kristianerik]

Added a note in HowToCrossCompileLLVM.rst specifying that CMake version 3.29 or later is required because the CMAKE_LINKER_TYPE variable used in the toolchain file is only supported starting with that version. Verified that the documentation builds correctly and the note displays as expected.

Fixes #165253

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