pythongh-115119: Recommend upstream libmpdec in build requirements (python#136205)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Author: 3 people

Doc/using/configure.rst

@@ -29,6 +29,9 @@ Features and minimum versions required to build CPython:
 
 * Tcl/Tk 8.5.12 for the :mod:`tkinter` module.
 
+* `libmpdec <https://www.bytereef.org/mpdecimal/doc/libmpdec/>`_ 2.5.0
+  for the :mod:`decimal` module.
+
 * Autoconf 2.72 and aclocal 1.16.5 are required to regenerate the
   :file:`configure` script.