Add goal debugging and graph inspection tools to documentation

[brandonwillard]

We need a documentation section that illustrates how to debug kanren goals and meta object unification (i.e. kanren.core.dbgo, and unification.debug_unify), as well as how to inspect graphs (i.e. theano.printing.debugprint and symbolic_pymc.tensorflow.printing.tf_dprint) and easily compare meta graphs (i.e. symbolic_pymc.utils.meta_diff).