chore(Algebra): move QuadraticAlgebra to QuadraticAlgebra/Defs (#31490)

Since we already have a `QuadraticAlgebra` folder, this should go in there. It looks strange on the documentation otherwise.

Author: themathqueen

Mathlib.lean

@@ -1084,8 +1084,8 @@ import Mathlib.Algebra.Polynomial.UnitTrinomial
 import Mathlib.Algebra.PresentedMonoid.Basic
 import Mathlib.Algebra.Prime.Defs
 import Mathlib.Algebra.Prime.Lemmas
-import Mathlib.Algebra.QuadraticAlgebra
 import Mathlib.Algebra.QuadraticAlgebra.Basic
+import Mathlib.Algebra.QuadraticAlgebra.Defs
 import Mathlib.Algebra.QuadraticDiscriminant
 import Mathlib.Algebra.Quandle
 import Mathlib.Algebra.Quaternion

Mathlib/Algebra/QuadraticAlgebra/Basic.lean

@@ -4,7 +4,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Antoine Chambert-Loir
 -/
 
-import Mathlib.Algebra.QuadraticAlgebra
+import Mathlib.Algebra.QuadraticAlgebra.Defs
 import Mathlib.Algebra.Star.Unitary
 import Mathlib.Tactic.FieldSimp