doc: application primer: improvements to "creating an application"
doc: application primer: improvements to "creating an application"

- Fix a few errors and bits of stale information in "Creating an
  Application", also adding additional cross-references.

- Consolidate some of the steps to help users create CMakeLists.txt
  correctly by unambiguously putting target_sources() after the
  boilerplate include.

- Split the description of the important build system variables into
  its own section and add more context for what they do.

- Add whitespace warning

Signed-off-by: Marti Bolivar <fd5da480b758a78208111e6ae5c6806e1f887137@foundries.io>

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