src/doc/en/reference/discrete_geometry/index.rst: Add backend_number_field

Author: Matthias Koeppe

src/doc/en/reference/discrete_geometry/index.rst

@@ -112,6 +112,7 @@ Backends for Polyhedra
    sage/geometry/polyhedron/backend_cdd
    sage/geometry/polyhedron/backend_cdd_rdf
    sage/geometry/polyhedron/backend_field
+   sage/geometry/polyhedron/backend_number_field
    sage/geometry/polyhedron/backend_normaliz
    sage/geometry/polyhedron/backend_polymake
    sage/geometry/polyhedron/backend_ppl