doc: kconfig: Move menuconfig/guiconfig docs to user guide

Move the documentation for the menuconfig and guiconfig interfaces from
Application Development to a new Interactive Kconfig Interfaces page in
User and Developer Guides.

Also tweak Application Development to mention the configuration
interfaces earlier. It's good to encourage people to try things out in
menuconfig, because dependencies often get lost when people seldom run
it and just hand-edit files.

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