maintainers: set gmarull as docs collaborator

@kartben has been doing a great job maintaining docs, so we're good in
this area. I'll stay as collaborator so I can still follow what's going
on.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

Author: gmarull

MAINTAINERS.yml

@@ -1034,9 +1034,9 @@ Documentation:
 Documentation Infrastructure:
   status: maintained
   maintainers:
-    - gmarull
     - kartben
   collaborators:
+    - gmarull
     - carlescufi
     - nashif
   files: