Subsectioned/rewrote Description, Added Julia links to Installation

[sbrincat]

Don’t want to step on anyone’s toes, but I thought the splash page could use a few changes:

• “About/Description” felt like kind of an intimidating big wall of text. I broke it into subsections: Overview, Features, Interface, Performance, and Implementation.
• To me, the Description felt a bit too oriented toward developers and algorithm aficionados, rather than end users. I moved all the stuff about what Julia packages NB uses under the hood to the end (Implementation section) and slightly trimmed some of the discussion of *DEs.
• “Installation” assumed users are good to go with Julia setup, but many/most will not be. I added links to install Julia and sign up for a JuliaHub account.

Of course, I’m the new guy here, so what do I know? So feel free to take these as suggestions and undo/amend as you see fit, especially if I’ve removed or altered something anyone sees as critical.

[harisorgn]

Thanks for the comments Scott. This looks good to me, it does make the intro page more inclusive to non-computational and non-Julian folks.