Fetch templates from Qt template folder when docs are made

Currently, all documentation projects that are located elsewhere than the Qt repository (for example, qtcreator\doc), must contain copies of the template files from the Qt repository (qt\doc\src\template) in their own doc\templates folder. This means that the templates can easily become out of date.

To make change management and troubleshooting easier by ensuring that all documentation projects always use the latest versions of the correct templates, the template files should be copied from the Qt repository to the doc\html\templates folder when the docs are built. Or they should be delivered in some other way that does not require manually copying them to the documentation project.