doc: Update for menuconfig.py

Update the Application Development Primer to describe the new menuconfig
configuration interface:

 - Update the section explaining how the configuration is navigated,
   changed, and saved

 - Add a section explaining how to jump directly to a symbol in the
   configuration interface

 - Replace mconf screenshots with menuconfig.py screenshots. Remove some
   screenshots that are no longer used.

 - Remove the section explaining how to load arbitrary .config files
   from within the configuration interface. That feature hasn't been
   implemented yet in menuconfig.py.

 - Do various minor language cleanup

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>