doc: toolchain: ac6: update documentation filename

This update changes the filename from 'arm_compiler.rst' to
'arm_compiler_6.rst' to accurately reflects its focus on AC6.

Recently, Arm introduced the Arm Toolchain for Embedded (ATfE), which is
distinct from AC6. This 7th generation compiler is entirely open source
for the first time. To distinguish it from AC6 and highlight it's a
complete toolchain rather than just a compiler, this new version will
be named Arm Toolchain for Embedded (ATfE).

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