MAINTAINERS: Add additional files to Bluetooth Audio

Neither samples nor documentation have a file structure
that allows us to easily add the Bluetooth Audio label
and reviewers to PRs that modify these files, so they
have been added more explicitly.

Any future files will need to be added to this list as
well, unless the structure change to make this easier.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>