Split DevGuide Into Pieces

* Gave each section its own file to make document easier to maintain and
  collaboratively review.  This should also fix the
  diffs-too-big-for-GitHub problem.

* Had to modify autocrossref.rb to search two different paths, as
  included files (for some reason) are built with a different working
  directory than primary files. The solution I used is a bit of a hack,
  but I was spending too much time trying to find The Right Way.

* DevRef has not yet been split. I have stashed changes to it which need
  to be finished and commited. Saivann is planning to work on it too, so
  I propose we leave it in one piece and split it in a few days.

It compiles correctly here and all three dev docs look correct to me.  I upload a preview because it should look identical to the previous version (including is done before markdown rendering).

I suggest merging as soon as Saïvann verifies it compiles correctly and looks good so we avoid unnecessary merge conflicts.