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 <marti@opensourcefoundries.com>