doc: kconfig: Put all documentation on setting symbols on a new page

Add a new 'Setting Kconfig configuration values' page in the new Kconfig
section in 'User and Developer Guides'. Move all the information on
setting Kconfig symbols from 'Application Development' and the board
porting guide into it. The same page now covers both configuration files
and Kconfig.defconfig files.

Add links to the new page in various places to make it easy to find.
Also add some more references to the top-level Kconfig page and other
Kconfig pages.

A lot of stuff was rewritten while moving it over (the CONF_FILE
documentation has been cleaned up in particular). Some new information
has been added as well, like a tip re. minimal configurations being
helpful when making Kconfig settings permanent, and a warning re.
dependencies being ORed rather than ANDed when defining a symbol in
multiple locations.

Fixes: #20915

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