Doc: Make C++20 overview visible to the new sidebar

Added the link to the qtcore-toc.

Task-number: QTBUG-130085
Pick-to: 6.9
Change-Id: I765e01aeda0bce694157bf15ff5a036f239168f5
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/doc/src/qtcore-toc.qdoc

@@ -36,6 +36,7 @@
             \li \l {Serializing Qt Data Types}
         \endlist
     \li \l {Container Classes}
+    \li \l {C++20 Overview}
     \li \l {Creating Custom Qt Types}
     \li \l {Qt's foreach Keyword}
     \li \l {Java-style Iterators in Qt}