Source tree, rupture set, and MFD documentation

Add additional documentation showing illustrations of source logic tree management (RuptureSet is tree 'leaf') and MFD tree management within a RuptureSet. Also describe how MFDs are collapsed.

Full epistemic: Map<Imt<Gmm, LogicTree<XySequence>>> --> Total: Map<Imt<Gmm, XySequence>>

Get images from nshmp-haz logic tree presentation(s)