Build 5.4 documentation snapshot

5.4 branch of Qt 5 is ready and there needs to be a snapshot of the documentation.

Involves:
-adding links to the 5.4 content in doc-snapshot.qt-project.org
-add 5.4 template with the '5.4' build version
-automate the process in Jenkins
-convert other projects such as Qt Creator to use QDoc in 5.4
-test the links