Add Short (400-Word) Introduction

* Describe the bare essentials to getting started plus how to use the
  guide's special features.

* I tweaked the noref regex to match any non-space character between the
  key and the noref tag, so we can easily prevent "terms" from being
  matched in strings like "terms-are-cool". For an example, see norefs
  next to `-testnet` strings in Intro section

* I didn't give the Introduction an h2 tag on purpose---I like jumping
  right in. However, I'm open to feedback.
Show crossref links on mobiles
Merge pull request #3 from saivann/devintro

Show crossref links on mobiles
Emphesize Advantages Of Regtest Mode (thanks @mikehearn!)
Merge branch 'intro' of https://github.com/harding/bitcoin.org into intro
Merge pull request #65 from harding/intro

Add Short (400-Word) Introduction