Open Medical Profile → Medical History. Select Add to Medical History, then Doctors, and then Continue. Under Type, specify whether you are a family doctor or specialist. Mandatory information is full name, street, zip code, city, country, state if applicable, and a phone number. A second address or phone number is optional.
After entering, select Save. If practice or contact details change, select Edit from the existing entry; with Delete you remove it. Doctors are not the same as Emergency contacts: Emergency contacts are trusted persons with their own invitation and confirmation.