New cheat sheet content

[cmutel]

Completely rewrote the "Getting Started" section:

• Now a single page cheat sheet focused on "how" - the "why" will come later in the "Overview" (will replace "Theory"). This helps me, at least, better understand what should go where.
• Removed a lot of obsolete or incorrect practice
• Made some changes to the core Brightway libraries when the API was dumb

[michaelweinold]

Sorry, but I do not see why this would all be in a single page. All the changes are fantastic - but borderline un-navigable. Also, the Section Navigation on the LHS is now completely empty.

I suggest to split up the file and re-populate the Section Navigation for easier navigation.