[committed][wwwdocs] Document libstdc++ changes in GCC 15

[Jonathan Wakely]

Pushed to wwwdocs.

---
 htdocs/gcc-15/changes.html | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/htdocs/gcc-15/changes.html b/htdocs/gcc-15/changes.html
index d0d6d147..91c020dd 100644
--- a/htdocs/gcc-15/changes.html
+++ b/htdocs/gcc-15/changes.html
@@ -84,7 +84,24 @@ a work-in-progress.</p>
     when parsing a template.
   </li>
 </ul>
-<!-- <h4 id="libstdcxx">Runtime Library (libstdc++)</h4> -->
+<h4 id="libstdcxx">Runtime Library (libstdc++)</h4>
+
+<ul>
+  <li>Improved experimental support for C++26, including:
+    <ul>
+    <li><code>views::concat</code>.</li>
+    <li>Member <code>visit</code>.</li>
+    <li>Type-checking <code>std::format</code> args.</li>
+    </ul>
+  </li>
+  <li>Improved experimental support for C++23, including:
+    <ul>
+    <li>
+      Clarify handling of encodings in localized formatting of chrono types.
+    </li>
+    </ul>
+  </li>
+</ul>
 
 <!-- <h3 id="d">D</h3> -->
 
-- 
2.46.0