MAINTAINERS: Add domains.py under West area

This small library is shared by both west and twister, where it's used
to parse the `domains.yaml` files generated by sysbuild. It also defines
the schema that sysbuild itself must adhere to when generating the file.

`domains.py` used to be located in `scripts/west_commands/`, which means
that it once belonged under "West". Having one file appear in multiple
areas is discouraged, so let's just move it back to "West".

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>