doc: application: make default app a workspace app

It is well known that putting your application inside the west
workspace where zephyr is installed, i.e. defining a workspace
application, makes it easier to use west build, since you don't have
to juggle setting ZEPHYR_BASE appropriately.

Therefore, recommend doing thing this way, while leaving a hint about
how to do something else.

The current state of affairs where the application is assumed to be in
$HOME/app is longstanding and precedes the introduction of west to
zephyr, and I think it's overdue for the page to get with the times
and use conventions that work well with west: our reference
application has been a workspace application for almost 2 years at
this point.

Create a new meta-variable <app> to describe the location of the
application to keep things short and make it clearer that the actual
location on the file system doesn't matter as long as things are set
up properly.

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