I wasn't quite liking the mingling of buildSubmorphs and updateRendering: I simplified it
added more comments about what the "updateRendering" method does.
in the attempt of keeping Morph as an abstract interface, I created a RectangleMorph...

…so whenever the simplest rectangle is needed, one can just call
RectangleMorph.
now creating the docs is completely disjoint from creating the build
rebuild and docs