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>