doc: Add a notice that you are free to choose compiler path

Works just fine if I point it to the default install path from
the Arm GCC installer which contains several spaces.
"C:\Program Files (x86)\GNU Arm Embedded Toolchain\x.x.x\"

The previous notice about not using spaces made sense to have this
very specific path but this might confuse people who install
ARM GCC with the provided installer which puts it into a path
under C:\Program FIles, to avoid people moving their installation
and possibly breaking a different setup I thinks its good
to make that clear that this is not a hard coded assumption
made across Zephyr and is just a matter of an ENV variable.

Signed-off-by: Timon Skerutsch <timon@diodes-delight.com>
Co-Authored-By: Stephanos Ioannidis <root@stephanos.io>