Technical Overview (including ios and android runtime)

[NathanWalker]

• https://v7.docs.nativescript.org/core-concepts/technical-overview
• https://v7.docs.nativescript.org/core-concepts/ios-runtime/overview
• https://v7.docs.nativescript.org/core-concepts/android-runtime/overview

[rigor789]

Merged these references into #47