ios: explain how to define icons and launch images, smaller fixes

- how to add icons and launch images in the .pro, so that one
  does not need to do it in Xcode in a brittle way that will
  be lost when the xcode project is regenerated,
- clarify how to use assets catalogs,
- clarify that creator can be used in some places.

Task-number: QTBUG-42049
Change-Id: Ia8b9d95526c0de4098c5f06eb5817dc7734f216e
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Doc: added brief statement to modules overview

Task-number: QTBUG-42682
Change-Id: I6167c7659f081a58f32195b7f2d3550e8fa7a379
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Add 5.4.0 changelog for qtdoc

Change-Id: I46b2ac71473aad572da62c624cd1f15e2daf4670
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Doc: Update external links to point to doc.qt.io

Task-number: QTBUG-43062
Change-Id: Idcca1ee428cbe1d51c6306ad4265f8dd32d80f2d
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Merge remote-tracking branch 'origin/5.4.0' into 5.4

Change-Id: I95c6aa408327f565606b7ee579becb6d07cc520d