[DOC] User Guide

[adam2392]

We should have a user-guide with high-level sections dedicated to the following:

1. What do we mean by networkx-compliant? Explain design philosophy and what classes are useful (i.e. MixedEdgeGraph) and how to use networkx causal algorithms vs networkx algorithms (i.e. by querying the sub-graph of specific edge types).
2. What is the interpretation of a causal graph? And link possibly relevant examples.
3. What is the interpretation of interventions in causal graphs? (i.e. F-nodes)
4. What is the interpretation of an equivalence class of graphs?

We should also have a glossary, where we define causal terms that might not be as readily accessible to new users: F-nodes, c-components, Markov equivalence class, SCM

[adam2392]

Added in #37