consolidate Cordova Push documentation

on the website we have two outdated tutorials on Cordova push:

• http://aerogear.org/docs/guides/aerogear-push-cordova-android/
• http://aerogear.org/docs/guides/aerogear-push-cordova-ios/

Besides that we have a short guide on our recent push plugin:

• http://aerogear.org/docs/guides/aerogear-cordova/AerogearCordovaPush/

That references to the above outdated docs for getting started.

We should get rid of the first two ones, and have "AerogearCordovaPush" doc contain the those links that the first two are containing (e.g. setting up APNs/GCM, Info on UPS etc)

It might be confusing if the outdate plugin docs are still around, and the recent doc is referencing to it, for a few steps (e.g. setup of APNs/GCM or UPS)