doc: add sxa as collaborator

Signed-off-by: Stewart Addison <sxa@uk.ibm.com>

PR-URL: https://github.com/nodejs/node/pull/34338


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Gus Caplan <me@gus.host>