Doc: rename index.html as extending-index.html

Remove the \contextpage commands, as they are only used
in some of the files.

Task-number: QTBUG-31106
Change-Id: Iacbf15a33ab6cb426c2fbfb6fbed03310095f2ba
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>

doc/api/coding-style.qdoc

@@ -17,7 +17,6 @@
 ****************************************************************************/
 
 /*!
-    \contentspage{index.html}{Qt Creator}
     \previouspage qtcreator-ui-text.html
     \page coding-style.html
     \nextpage qtcreator-api.html

doc/api/external-tool-spec.qdoc

@@ -17,7 +17,6 @@
 ****************************************************************************/
 
 /*!
-    \contentspage{index.html}{Qt Creator}
     \page external-tool-spec.html
     \nextpage coding-style.html
 

doc/api/qtcreator-api.qdoc

@@ -17,7 +17,6 @@
 ****************************************************************************/
 
 /*!
-    \contentspage{index.html}{Extending Qt Creator Manual}
     \page qtcreator-api.html
 
     \title Qt Creator API Reference

doc/api/qtcreator-dev.qdoc

@@ -17,7 +17,7 @@
 ****************************************************************************/
 
 /*!
-    \page index.html
+    \page extending-index.html
     \title Extending Qt Creator Manual
 
     Qt Creator is a cross-platform integrated development environment (IDE)

doc/api/qtcreator-documentation.qdoc

@@ -17,7 +17,6 @@
 ****************************************************************************/
 
 /*!
-    \contentspage{index.html}{Qt Creator}
     \previouspage external-tool-spec.html
     \page qtcreator-documentation.html
     \nextpage coding-style.html

doc/api/qtcreator-ui-text.qdoc

@@ -17,7 +17,6 @@
 ****************************************************************************/
 
 /*!
-    \contentspage{index.html}{Qt Creator}
     \previouspage external-tool-spec.html
     \page qtcreator-ui-text.html
     \nextpage coding-style.html