Doc: Remove outdated "Add-ons" page

The list is outdated with the emergence of new modules. There's already
an up-to-date list in qtmodules.qdoc; it's better to maintain one list.

Some unique text from the "Add-ons" page is moved to the "All Modules"
page, with modifications.

Links removed without replacement:
- topics.qdoc: Add-ons are not a programming topic
- datastorage.qdoc: There are no add-ons related to data storage or
  compression

Change-Id: I40abc6a7f5631c4e192f5754b5fcf1b24288db63
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>