I want to know is there a schematic of the SHIELD-EKG-EMG-PA electrode, user manual or technical documents for it
No, but you can inspect the female audio jack named CH_IN of the SHIELD-EKG-EMG - this would help you understand the connections of the male audio jack of the electrode (where L, R, and GND should be attached).
The schematics are here: https://www.olimex.com/Products/Duino/Shields/SHIELD-EKG-EMG/resources/SHIELD-EKG-EMG-REV-B-SCHEMATIC.pdf
Best regards,
Lub/OLIMEX