doc: update KConfig style guidelines

Add/update guidelines on how KConfig files should be written and organized
by identifying a common style currenlty used in most areas of the project
and formalizing it.

This should provide an aid for future merge request, code reviews and
refactorings to enable the acting party to better enforce consistency
in KConfig files.

Signed-off-by: Tim Schrader <tim.schrader92@gmail.com>