gh-97913 Docs: Add walrus operator to the index (GH-97921)


* Add walrus operator to the index

* Add named expression to the index

Co-authored-by: Mariatta Wijaya <Mariatta@users.noreply.github.com>

* Fix indentation and add missing newline

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>

Co-authored-by: Mariatta Wijaya <Mariatta@users.noreply.github.com>
Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>
(cherry picked from commit 296313002fde56f52d6c81f17d7ba5c2eb57d098)

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index 1e76b25..31cdc5c 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -1723,6 +1723,12 @@
 (for example, ``not 'foo'`` produces ``False`` rather than ``''``.)
 
 
+.. index::
+   single: := (colon equals)
+   single: assignment expression
+   single: walrus operator
+   single: named expression
+
 Assignment expressions
 ======================