Improve Initial Docs Experience

[justsml]

Hi y'all, thanks to everyone for all the work that's gone into this library!!! 👍

Here's a summary of my progress so far:

• Add simpler "basic" example for "Quick Start."
• Enhance (previous) nested routing example from "Quick Start."
• Add explainer text to examples.
• Move Philosophy to lower in the nav.
• Quick Start is default content.

RE: #6409

[justsml]

For completeness, here's the before and after screenshots:

Before

After

[justsml]

Thanks for the 👍 @pshrmn @timdorr

[mjackson]

Thanks for the PR, @justsml! This looks great. I'll deploy it now.

[mjackson]

Should deploy as soon as https://travis-ci.org/ReactTraining/react-router/builds/445992237 is finished 👍

[justsml]

That's great news @mjackson!!!
Thanks for the merge! 👍

[mjackson]

Thank you for your contributions to the docs, @justsml :) A lot of people are going to benefit from the improvements you're making.