doc: enhance cross compile section in getting started
doc: enhance cross compile section in getting started

"Getting Started" does not clearly state how to use a custom cross
compiler.  This patch adds a new section "Using Custom Cross
Compilers" to it.

To support users without SDK, we have the section "Building without
the Zephyr SDK".  But the section is all about prebuilt tools in the
SDK but not build process.  This patch adds two pointers "Using 3rd
Party Cross Compilers" and "Using Custom Cross Compilers" pointing to
the actual building process, and let those section describe
ZEPHYR_TOOLCHAIN_VARIANT, which is cross compilers dependent variable.

The patch also clarify what a 3rd party cross compiler is and what a
custom compiler is.

Signed-off-by: Yasushi SHOJI <b0fcb18d68856f3f248fde2125e5497d662c9efd@ispace-inc.com>

GiulianoFranchetto/zephyr,galak/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,galak/zephyr,explora26/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,ldts/zephyr,punitvara/zephyr,punitvara/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,punitvara/zephyr,nashif/zephyr,finikorg/zephyr,ldts/zephyr,nashif/zephyr,punitvara/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,explora26/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,explora26/zephyr,kraj/zephyr,nashif/zephyr,explora26/zephyr,ldts/zephyr,ldts/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,kraj/zephyr