mathlib3

19786714 - feat(category_theory/triangulated): changing namespaces, introducing triangulated categories (#17284)

feat(category_theory/triangulated): changing namespaces, introducing triangulated categories (#17284) This PR moves the namespace `category_theory.triangulated.pretriangulated` to `category_theory.pretriangulated`, and introduces triangulated categories. Co-authored-by: Joël Riou <37772949+joelriou@users.noreply.github.com>