mathlib3

3ac4bb24 - feat(combinatorics/simple_graph/basic): add definition of complement of simple graph (#5697)

feat(combinatorics/simple_graph/basic): add definition of complement of simple graph (#5697) Add definition of the complement of a simple graph. Part of branch [strongly_regular_graph](https://github.com/leanprover-community/mathlib/tree/strongly_regular_graph), with the goal of proving facts about strongly regular graphs. Co-authored-by: agusakov <39916842+agusakov@users.noreply.github.com>