doc: toolchain: atfe: arm toolchain for embedded documentation

Arm Toolchain for Embedded (ATfE) is the next-generation Arm embedded
C/C++ compilation toolchain.

Arm Toolchain for Embedded is 100% open source. Arm is investing in the
LLVM linker and minor tools, and in the Picolib C library, to create a
performant 100% open source compilation toolchain for embedded
development with Arm-based designs.

The proprietary components used in Arm Compiler for Embedded 6 (the
linker, C library, and binutils carried over from the legacy Arm
Compiler 5 toolchain) are retired in favour of their open source
(LLVM and Picolib) equivalents.

Product page: https://developer.arm.com/Tools%20and%20Software/Arm%20Toolchain%20for%20Embedded
Github repo: https://github.com/arm/arm-toolchain



Signed-off-by: Alex Fabre <alex.fabre@rtone.fr>