CODEOWNERS: add @sebastiaanspeck to codeowners (#11076)

.github/CODEOWNERS

@@ -2,6 +2,7 @@
 /pages.es/ @navarroaxel @kant
 /pages.id/ @reinhart1010
 /pages.it/ @mebeim @yutyo
+/pages.nl/ @sebastiaanspeck
 /pages.pl/ @acuteenvy
 /pages.pt_BR/ @waldyrious @isaacvicente
 /pages.pt_PT/ @waldyrious
@@ -14,6 +15,7 @@
 /contributing-guides/*.es.md @navarroaxel @kant
 /contributing-guides/*.id.md @reinhart1010
 /contributing-guides/*.it.md @mebeim @yutyo
+/contributing-guides/*.nl.md @sebastiaanspeck
 /contributing-guides/*.pl.md @acuteenvy
 /contributing-guides/*.pt_BR.md @waldyrious @isaacvicente
 /contributing-guides/*.pt_PT.md @waldyrious