first pass at fleshing out the guide

Given the nature of this PR, it might be easier to review/comment on a rendered version of the content. I've created a google doc out of this page:
https://docs.google.com/document/d/1vR1acWGkUWPw5L5mvAh8tRg0UY_bB8H-Ajfv7pd8fSg/edit#
(It's not a perfect copy.)
I talked with Eric about this some. The goals is to present conditional loading here, but not really get into the topic. Delegate that to the docs of the conditional loading libraries/services.

We could perhaps choose a better library/service (or a list of them). Any ideas?