Update 1.2 Introduction

[dcmiddle]

I don't know what you want the directory organization to be but it looks like most things look nested under the doc version like: docs/1.2/transaction_family_specification/index.md but introduction is under core/version docs/core/1.2/introduction.md

[agunde406]

It will be moved after https://github.com/hyperledger/sawtooth-docs/pull/100 goes in

[agunde406]

Items are going to be moved incrementally