[3.13] gh-120426: Add the glossary term "immortal" (GH-123191) (#123491)

Add the glossary term "immortal", mark it as an implementation detail

(cherry picked from commit 6754566)
(cherry picked from commit 1af74fa)

(Unlike the original commits, this adds the entire entry.)

Doc/glossary.rst

@@ -591,6 +591,14 @@ Glossary
       :ref:`idle` is a basic editor and interpreter environment
       which ships with the standard distribution of Python.
 
+   immortal
+      *Immortal objects* are a CPython implementation detail introduced
+      in :pep:`683`.
+
+      If an object is immortal, its :term:`reference count` is never modified,
+      and therefore it is never deallocated while the interpreter is running.
+      For example, :const:`True` and :const:`None` are immortal in CPython.
+
    immutable
       An object with a fixed value.  Immutable objects include numbers, strings and
       tuples.  Such an object cannot be altered.  A new object has to