doc: add slog to release notes

Updates #58645

Change-Id: Ice8f115f00c62dcffd0c7b78bb8a7d66d832075d
Reviewed-on: https://go-review.googlesource.com/c/go/+/496194
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>

doc/go1.21.html

@@ -154,6 +154,15 @@ <h2 id="linker">Linker</h2>
 
 <h2 id="library">Core library</h2>
 
+<h3 id="slog">New log/slog package</h3>
+
+<p>
+  The new <a href="/pkg/log/slog">log/slog</a> package provides structured logging with levels.
+  Structured logging emits key-value pairs
+  to enable fast, accurate processing of large amounts of log data.
+  The package supports integration with popular log analysis tools and services.
+</p>
+
 <h3 id="minor_library_changes">Minor changes to the library</h3>
 
 <p>