Fix broken link to Zstandard library

The old link zstd.net is not working any more. Zstandard is now at
http://facebook.github.io/zstd/. To ease maintenance in the future,
those links now point to "Zstandard Site" which is maintained in
external-resources.qdoc.

Task-number: QTBUG-88533
Pick-to: 6.0
Change-Id: Ic8f067fd5d7ce1a088d0272797fca98fd506a26e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

doc/global/externalsites/external-resources.qdoc

@@ -527,3 +527,8 @@
     \externalpage https://login.qt.io/
     \title Qt Account Sign-up
 */
+
+/*!
+   \externalpage http://facebook.github.io/zstd/
+   \title Zstandard Site
+/

src/corelib/doc/src/resource-system.qdoc

@@ -215,7 +215,7 @@
       XML file to indicate a file should be most compressed, regardless of
       which algorithms \c rcc supports.
 
-      \li \c{zstd}: use the \l{https://zstd.net}{Zstandard} library to compress
+      \li \c{zstd}: use the \l{Zstandard Site}{Zstandard} library to compress
       contents. Valid compression levels range from 1 to 19, 1 is least
       compression (least CPU time) and 19 is the most compression (most CPU
       time). The default level is 14. A special value of 0 tells the \c{zstd}

src/corelib/io/qresource.cpp

@@ -284,7 +284,7 @@ static inline QStringList *resourceSearchPaths()
     \value NoCompression    Contents are not compressed
     \value ZlibCompression  Contents are compressed using \l{https://zlib.net}{zlib} and can
                             be decompressed using the qUncompress() function.
-    \value ZstdCompression  Contents are compressed using \l{https://zstd.net}{zstd}. To
+    \value ZstdCompression  Contents are compressed using \l{Zstandard Site}{zstd}. To
                             decompress, use the \c{ZSTD_decompress} function from the zstd
                             library.