docs/doc.md: Don't link to developer-old.gnome.org

That web site has been removed.

docs/doc.md

@@ -5,11 +5,7 @@ title: Documentation
 
 ## libsigc++-2.0
 
-We have [tutorial-style](https://developer-old.gnome.org/libsigc++-tutorial/2.10/)
-and [reference](https://developer-old.gnome.org/libsigc++/2.10/) documentation.
-
-This documentation is frozen on the web. It does not document the latest release.
-If you want newer documentation, you can download a tarball from
+The documentation is not available online. You can download a tarball from
 [GitHub releases](https://github.com/libsigcplusplus/libsigcplusplus/releases/)
 or the [GNOME download site](https://download.gnome.org/sources/libsigc++/),
 extract it, and view the documentation at *untracked/docs/*.