Factor out developer guide into separate files by section

[qualidafial]

Now that the developer guide structure has had a chance to settle, it's a good time to factor it back out into different files by section to make it easier to manage going forward.

[stevenschlansker]

Note that I believe the reason we did not do this in the first place was because it made it difficult to generate a Table of Contents or produce an all-in-one doc -- it'd be nice to ensure that doesn't break when we split it up again