mathlib

c3f4d1b8 - refactor(combinatorics/simple_graph): move simple graph files to their own folder (#5154)

refactor(combinatorics/simple_graph): move simple graph files to their own folder (#5154) Move the files into one folder with the goal of integrating material from the branch [simple_graphs2](https://github.com/leanprover-community/mathlib/tree/simple_graphs2)