doc: add @nodejs/documentation to CC table

PR-URL: #13952
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/onboarding-extras.md

@@ -6,6 +6,7 @@
 | --- | --- |
 | `benchmark/*` | @nodejs/benchmarking, @mscdex |
 | `bootstrap_node.js` | @fishrock123 |
+| `doc/*`, `*.md` | @nodejs/documentation |
 | `lib/assert` | @nodejs/testing |
 | `lib/buffer` | @nodejs/buffer |
 | `lib/child_process` | @bnoordhuis, @cjihrig |