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>

Author: vsemozhetbyt

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|