Doc: link to "Which string class to use?" on all string pages

This section has very useful flowcharts. The question "Which string
class to use?" is exactly what most users will want the answer to when
faced with all of the different string types we provide.

Task-number: QTBUG-135947
Change-Id: I2ae79803e205693d6c7ee8f50cad5d057c4d24ea
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/text/qanystringview.cpp

@@ -103,7 +103,7 @@ QT_BEGIN_NAMESPACE
     and UTF-8 multibyte sequences count as two, three or four,
     depending on their length).
 
-    \sa QUtf8StringView, QStringView
+    \sa {Which string class to use?}, QUtf8StringView, QStringView
 */
 
 /*!

src/corelib/text/qstring.cpp

@@ -2132,7 +2132,8 @@ void qtWarnAboutInvalidRegularExpression(const QString &pattern, const char *whe
     Mitigating or controlling the behavior these limits cause is beyond the
     scope of the Qt API.
 
-    \sa fromRawData(), QChar, QStringView, QLatin1StringView, QByteArray
+    \sa {Which string class to use?}, fromRawData(), QChar, QStringView,
+        QLatin1StringView, QByteArray
 */
 
 /*! \typedef QString::ConstIterator

src/corelib/text/qstringview.cpp

@@ -67,7 +67,7 @@ QT_BEGIN_NAMESPACE
     allowed in \c constexpr functions). You can use an indexed loop and/or utf16() in
     \c constexpr contexts instead.
 
-    \sa QString
+    \sa {Which string class to use?}, QString
 */
 
 /*!