Doc: replace \target with \keyword if at start of page

A \target whose purpose is to link to the top of a
page (and not to a section within a page) works better
as a \keyword, because \target generates a
new html anchor which, in this case, is not tied to
any title element on the page.
A \keyword links to the page itself, as expected.

Task-number: QTBUG-48482
Change-Id: I957551edd0eb7e665358d04b37dab41e2686b851
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>

qmake/doc/src/qmake-manual.qdoc

@@ -4466,7 +4466,7 @@
 */
 
 /*!
-    \target qmake-getting-started
+    \keyword qmake-getting-started
     \page qmake-tutorial.html
     \title Getting Started
     \contentspage {qmake Manual}{Contents}

src/corelib/doc/src/objectmodel/metaobjects.qdoc

@@ -32,7 +32,7 @@
 
     \ingroup qt-basic-concepts
     \keyword meta-object
-    \target Meta-Object System
+    \keyword Meta-Object System
 
     Qt's meta-object system provides the signals and slots mechanism for
     inter-object communication, run-time type information, and the dynamic

src/corelib/doc/src/objectmodel/properties.qdoc

@@ -31,7 +31,7 @@
     \brief An overview of Qt's property system.
 
     \ingroup qt-basic-concepts
-    \target Qt's Property System
+    \keyword Qt's Property System
 
     Qt provides a sophisticated property system similar to the ones
     supplied by some compiler vendors. However, as a compiler- and

src/corelib/global/qnamespace.qdoc

@@ -28,7 +28,7 @@
 /*!
     \namespace Qt
     \inmodule QtCore
-    \target Qt Namespace
+    \keyword Qt Namespace
 
     \brief The Qt namespace contains miscellaneous identifiers
     used throughout the Qt library.

src/corelib/plugin/qplugin.qdoc

@@ -28,7 +28,7 @@
 /*!
     \headerfile <QtPlugin>
     \title Defining Plugins
-    \target qtplugin-defining-plugins
+    \keyword qtplugin-defining-plugins
     \ingroup plugins
 
     \brief The <QtPlugin> header file defines macros for defining plugins.

src/dbus/doc/src/qtdbus-module.qdoc

@@ -34,7 +34,7 @@
     \ingroup modules
     \qtvariable dbus
 
-    \target The QDBus compiler
+    \keyword The QDBus compiler
 
 
     Applications using the Qt D-Bus module can provide services to

src/tools/qdoc/doc/qdoc-minimum-qdocconf.qdoc

@@ -26,7 +26,7 @@
 ****************************************************************************/
 /*!
 \page qdoc-minimum-qdocconf.html
-\target minimal-qdocconf
+\keyword minimal-qdocconf
 \title A Minimal qdocconf File
 
 \brief Describes a minimal .qdocconf file

src/widgets/doc/src/qtwidgets-examples.qdoc

@@ -127,7 +127,7 @@
 
 /*!
     \ingroup all-examples
-    \target Graphicsview Examples
+    \keyword Graphicsview Examples
     \title Graphics View Examples
     \brief Using the Graphics View framework.
     \page examples-graphicsview.html