updated documentation for aerogear mobile-docs

[dmak84]

updated documentation for aerogear mobile-docs

• updated Data Sync Native SDK native/ds-android-intro.html, for both Android and iOS, added reference link to Apollo Client.
• added description of mobile-service.json file in identity-management.html#adding-user-authentication-to-your-mobile-app
• added Push notification section for registering and unregistering a device, we added the REST section push-notifications.html#registering-device
• added docs for the Mobile Security REST API security-api.html, the REST API is based on the swagger.yaml file https://github.com/aerogear/mobile-security-service/blob/master/api/swagger.yaml