Update package installation to reflect Studio app def

Bertrand, can you plan to update this page : https://doc.nuxeo.com/nxdoc/installing-a-new-package-on-your-instance/ (edited)

to explain that a package can be installed manually like this, but that people should also give a look at the application definition notion, that is the recommended way for packages that are part of the permanent set up
(with a link to the application definition studio screen documentation).