doc: add legacy status to stability index

Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/37784


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>