docs(discretization/domain/schema/validation): updated docs and brought some imports over from the new backend/mfem

src/include/serif/discretization/domain/schema/validation/mesh.hpp

@@ -0,0 +1,7 @@
+#pragma once
+
+#include "serif/backend/mfem/discretization/schema.hpp"
+
+namespace serif::discretization::domain::schema::validation {
+    using serif::backend::mfem::discretization::validate_schema;
+}