pythongh-97913 Docs: Add walrus operator to the index (python#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>

Doc/reference/expressions.rst

@@ -1741,6 +1741,12 @@ returns a boolean value regardless of the type of its argument
 (for example, ``not 'foo'`` produces ``False`` rather than ``''``.)
 
 
+.. index::
+   single: := (colon equals)
+   single: assignment expression
+   single: walrus operator
+   single: named expression
+
 Assignment expressions
 ======================