doc: zephyr build process description

Updated the Zephyr build process description to include description of
all Zephyr link stages.

It extends the guide with description of the intermediate binaries as
well as the post-processing of those binaries.
- Unfixed size binary
- Fixed size binary

Post-processing
- Partition alignment
- ISR table generation

Build stages images has been updated and new images showing the
post-processing stages of the intermediate binaries has been added.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>