Add information about example code to Quarkus project generator procedure

Update Step 3, substep iii of the procedure about using code.quarkus.redhat.com

The package name field describe in the substep is no longer in the UI, and an option to add starter code for some extensions has been added to the additional options.
Rewrite step 3 to reflect the updates.

The related image has been updated in QUARKUS-916, but the text must be edited, too.