[gnomemm-website] libsigc++ has moved to GitHub

[Kjell Ahlstedt <kjellahl src gnome org>]

commit e22d327588efd8e84dd34db499e0cc0f7013a9bd
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed Jan 5 16:05:07 2022 +0100

    libsigc++ has moved to GitHub

 docs/C/documentation.xml | 2 +-
 docs/C/download.xml      | 2 +-
 docs/C/main.xml          | 4 ++--
 3 files changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/docs/C/documentation.xml b/docs/C/documentation.xml
index 9f5780b..377c78a 100644
--- a/docs/C/documentation.xml
+++ b/docs/C/documentation.xml
@@ -156,7 +156,7 @@
             <td>
               <itemizedlist>
                 <listitem>
-                  <para><ulink url="http://libsigc.sourceforge.net/doc.shtml";>libsigc++ 
documentation</ulink></para>
+                  <para><ulink url="https://libsigcplusplus.github.io/libsigcplusplus/";>libsigc++ 
website</ulink></para>
                 </listitem>
                 <listitem>
                   <para><ulink url="https://developer.gnome.org/gtk3/stable/";>GTK+ reference</ulink></para>
diff --git a/docs/C/download.xml b/docs/C/download.xml
index b5bb057..2ddc66b 100644
--- a/docs/C/download.xml
+++ b/docs/C/download.xml
@@ -75,7 +75,7 @@ operating system unless you wish to test the latest versions. However, <ulink ur
               <para><ulink url="http://www.gtk.org";>GTK+</ulink></para>
             </listitem>
             <listitem>
-              <para><ulink url="http://libsigc.sourceforge.net";>libsigc++</ulink></para>
+              <para><ulink url="https://libsigcplusplus.github.io/libsigcplusplus/";>libsigc++</ulink></para>
             </listitem>
           </itemizedlist></para>
       </sect2>
diff --git a/docs/C/main.xml b/docs/C/main.xml
index 36a6f43..ea1701d 100644
--- a/docs/C/main.xml
+++ b/docs/C/main.xml
@@ -83,7 +83,7 @@
    gtkmm is a GUI toolkit and nothing more, and it strives to be the
    best C++ GUI toolkit. That mandate does not cover generic C++ data
    structures, an XML parser, or database access. Even our signal/slots
-   system is part of a separate (<ulink url="http://libsigc.sourceforge.net";>libsigc++</ulink>) library.
+   system is part of a separate (<ulink 
url="https://libsigcplusplus.github.io/libsigcplusplus/";>libsigc++</ulink>) library.
    However, gtkmm is designed to work well with all other C++ libraries.
    </para>
         <para>
@@ -94,7 +94,7 @@
       <sect2>
         <title>libsigc++</title>
         <para>
-    Also, be sure to check out our companion libraries, glibmm (UTF8 strings, threading, etc) and <ulink 
url="http://libsigc.sourceforge.net/";>libsigc++</ulink> (signal/slots).
+    Also, be sure to check out our companion libraries, glibmm (UTF8 strings, threading, etc) and <ulink 
url="https://libsigcplusplus.github.io/libsigcplusplus/";>libsigc++</ulink> (signal/slots).
    </para>
       </sect2>
       <sect2>