doc: genrest: Separate turbo mode logic from rest of code
doc: genrest: Separate turbo mode logic from rest of code

KCONFIG_TURBO_MODE being twisted up in the normal logic made the code
hard to follow and change.

Use a separate write_dummy_index() function for KCONFIG_TURBO_MODE
instead, and add more documentation for it. Also get rid of options.rst
and just write all the symbol link targets directly to the dummy index
file, which is a bit simpler.

As a small piggybacked improvement for default values, make the heading
'default' instead of 'defaults' when there's only one. The menuconfigs
do this too.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr