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