[3.14] Link to 3.14 GC design docs

The devguide link is dead and redirects to the CPython internal docs on
main. So let's just link directly to the CPython internal docs on 3.14

Author: hauntsaninja

Doc/library/gc.rst

@@ -140,7 +140,7 @@ The :mod:`gc` module provides the following functions:
 
    *threshold2* is ignored.
 
-   See `Garbage collector design <https://devguide.python.org/garbage_collector>`_ for more information.
+   See `Garbage collector design <https://github.com/python/cpython/blob/3.14/InternalDocs/garbage_collector.md>`_ for more information.
 
    .. versionchanged:: 3.14
       *threshold2* is ignored