doc: add `String.prototype.at` and `%TypedArray%.prototype.at`

PR-URL: https://github.com/nodejs/node/pull/39583


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Ash Cripps <acripps@redhat.com>