Doc: Fix warnings about snippets in qtdeclarative

Cannot find file to quote from: 'code/backend/backend.pro'
Cannot find file to quote from: 'code/doc_src_qtqml.pro'
Cannot find file to quote from: 'code/doc_src_qtquick.pro'

Pick-to: 6.1
Change-Id: I26642a375a659a3d8dbda097702ffc2f68d10137
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/qml/doc/src/cppintegration/topic.qdoc

@@ -53,8 +53,11 @@ The \c QML_ELEMENT macro makes the BackEnd class available in QML.
 
 \li Add the following lines to your project file:
 
-\snippet code/backend/backend.pro registration
-
+\badcode
+CONFIG += qmltypes
+QML_IMPORT_NAME = io.qt.examples.backend
+QML_IMPORT_MAJOR_VERSION = 1
+\endcode
 The BackEnd class is automatically registered as a type, which is accessible
 from QML by importing the URL, "\c{io.qt.examples.backend 1.0}".
 

src/qml/doc/src/qtqml.qdoc

@@ -75,7 +75,9 @@ See also the \l{Build with CMake} overview.
 
 To configure the module for building with qmake, add the module as a value of the QT variable in the project's .pro file:
 
-\snippet code/doc_src_qtqml.pro 0
+\badcode
+QT += qml
+\endcode
 
 \section1 QML and QML Types
 

src/quick/doc/src/qtquick-cpp.qdoc

@@ -43,7 +43,9 @@
   To link against the module, add this line to your \l qmake \c
   .pro file:
 
-  \snippet code/doc_src_qtquick.pro 0
+  \badcode
+  QT += quick
+  \endcode
   \endif
 
   For more information on the Qt Quick module, see the

src/quick/doc/src/qtquick.qdoc

@@ -81,7 +81,9 @@ See also the \l{Build with CMake} overview.
 To configure the module for building with qmake, add the module as a value of
 the QT variable in the project's .pro file:
 
-\snippet code/doc_src_qtquick.pro 0
+\badcode
+QT += quick
+\endcode
 
 \section1 Important Concepts in Qt Quick