Applies new theme and adds docs link to template

Created by: lukejacksonn

I was inspired by the new react docs, to readdress https://github.com/facebookincubator/create-react-app/pull/1472 and give App.js some love and attention; changing as little as possible but embracing the new theme and adding a bit of responsiveness.

🙂 BEFORE

😎 AFTER

💅 CHANGES

• Updates .App-header background-color to #282c34 from reactjs.org
• Moves .App-intro into .App-header and added min-height: 100vh to header
• Adds .App-link CTA highlighted in #61daFb which links to reactjs.org for reference
• Removes .App-title and To get started, text in .App-intro for the sake of brevity
• Aligns .App-header contents vertically and horizontally using display: flex
• Optimises screen fit using font-size: calc(10px + 2vmin) on .App-header
• Increases the size of the rotating icon for dramatic effect

💭 THOUGHTS

One concern I have about this design is that if a first time user goes to App.js and enters content below .App-header then it won't be immediately visible due to the header being 100vh. A solution to this would be setting the height of the header to 90vh and setting the background colour of .App or body to #20232a (the darker grey in the theme).

The changes, although superficial, make the first creat-react-app experience a little more magical and a little more useful. It is in keeping with the react theme and portrays a truer representation of the products build quality and underlying technologies.

I'm aware this PR is massively subjective but let me know what you think. I am more than willing to discuss/make changes if you like the general direction!