gdbus: convert doc/supplementaryservices-api.txt

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gdbus: convert doc/text-telephony-api.txt

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gdbus: convert doc/voicecall-api.txt

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gdbus: convert doc/voicecallmanager-api.txt

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>