Documentation page to give overview of core logic

[liz-is]

We discussed adding separate pages to clarify some aspects of the core logic of ReX (and diagrams). The key pages/sections I think we'd need are:

• Mutant generation, and how the tree of mutants works, to help explain tree_depth, search_limit, etc
• Something to explain how queue_style works
• How the global method for finding an explanation works - explaining chunk_size
• How the spatial method for finding an explanation works - explaining initial_radius, radius_eta, etc
• (Later) How multi-explanations work, including the obj_function option.

It'd also be nice (but lower priority) to have a page/section about the evaluation metrics with e.g. links to papers defining/using these.