doc: flash_partitions: rework
doc: flash_partitions: rework

Re-work the flash partitions documentation:

- add sections to provide more complete context

- link to useful information on DT overlays elsewhere in the
  documentation

- touch up some phrasing

- inline a smaller, self-contained example within the page rather than
  linking to an entire board dts

Signed-off-by: Marti Bolivar <fd5da480b758a78208111e6ae5c6806e1f887137@opensourcefoundries.com>

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