chore(NumberTheory/Height): move Instances.lean -> NumberField.lean (leanprover-community#34776)

This PR just changes the name of "Instances.lean" to "NumberField.lean".

Rationale: I expect to add quite a number of number field specific results on heights eventually, and it makes sense to put them together with the `AdmissibleAbsValues` instance for number fields. Further instances for function fields can be added in a separate "FunctionField.lean" file at some later point.

The module docstring will be updated in a follow-up PR.

Author: MichaelStollBayreuth

Mathlib.lean

@@ -5328,7 +5328,7 @@ public import Mathlib.NumberTheory.Harmonic.GammaDeriv
 public import Mathlib.NumberTheory.Harmonic.Int
 public import Mathlib.NumberTheory.Harmonic.ZetaAsymp
 public import Mathlib.NumberTheory.Height.Basic
-public import Mathlib.NumberTheory.Height.Instances
+public import Mathlib.NumberTheory.Height.NumberField
 public import Mathlib.NumberTheory.JacobiSum.Basic
 public import Mathlib.NumberTheory.KummerDedekind
 public import Mathlib.NumberTheory.LSeries.AbstractFuncEq