doc: application development: adjust creation section hierarchy

The 'Creating an Application' section has existed for several years
and predates the introduction of the example-application repository.

It's still a good reference, but it's not really the easiest way to
make an applications any more. Judging by experience watching users
ask questions and receive support on Discord, the example-application
repository is serving its purpose as a better 'pre-cooked' starting
point.

Adjust the hierarchy so that there's a single, parent section about
creating applications, which has using example-application as one
alternative, and doing it by hand as a less-recommended option. Add
more text on exactly what you need to do with example-application to
get something you can actually use.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>