doc: application: improve example-application section

The 'Example Application' text is similar to but not the same as the
name of the repository, which is example-application. Use the name of
the repository instead. This is easier to search for and plants seeds
in people's memory about where to find it on GitHub.

Add more cross references.

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