MAINTAINERS: add nanopb documentation to nanopb area

adds doc/services/serialization/nanopb.rst to nanopb area

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

MAINTAINERS.yml

@@ -5724,6 +5724,7 @@ West:
   maintainers:
     - pdgendt
   files:
+    - doc/services/serialization/nanopb.rst
     - modules/nanopb/
     - samples/modules/nanopb/
     - tests/modules/nanopb/