Move documentationPath() from Core::ICore to Core::HelpManager

It is documentation related API, so it belongs there.

Change-Id: I5d1676f251e6deb92050ddedac19bf3c332aab54
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/coreplugin/icore.cpp

@@ -445,11 +445,6 @@ QString ICore::installerResourcePath()
            + Constants::IDE_ID;
 }
 
-QString ICore::documentationPath()
-{
-    return QDir::cleanPath(QCoreApplication::applicationDirPath() + '/' + RELATIVE_DOC_PATH);
-}
-
 /*!
     Returns the path to the command line tools that are shipped with \QC (corresponding
     to the IDE_LIBEXEC_PATH qmake variable).