doc: kconfig: Touch up preprocessor docs and put it on a separate page

Touch up the Kconfig preprocessor function documentation a bit and add
some more details to the example. Use the Kconfig preprocessor call
syntax instead of Python syntax in the function synopses.

Also move the preprocessor docs to a separate page, and add a top-level
Kconfig section to User and Developer Guides that links to the Kconfig
tips page and the preprocessor page.

Trying to avoid the Kconfig tips page becoming a dumping ground for
random Kconfig documentation.

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