Documented the MXChip Newlib runtime glue

[fdesbiens]

Summary

• Explain why the MXChip sample supplies Newlib and Newlib-nano system-call hooks.
• Identify the console implementations supplied elsewhere in the sample.
• Distinguish this toolchain and BSP integration from ThreadX and its POSIX compatibility layer.
• Document the limitations of the sample heap implementation and other minimal stubs.

Related to eclipse-threadx/threadx#697.

Validation

• git diff --check
• MXChip starter configuration built successfully with CMake, Ninja, and Arm GNU Toolchain 13.3.Rel1.