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>