Add goal debugging and graph inspection tools to documentation

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`).