DOCS: Undo ignoring `FormArgument` refs in docs

[connorjward]

In #4537 we disabled Sphinx referencing for ufl.core.terminal.FormArgument as it was raising the following warnings:

writing output... [ 99%] visualisation
writing output... [100%] zenodo
/__w/firedrake/firedrake/venv/lib/python3.12/site-packages/firedrake/cofunction.py:docstring of firedrake.cofunction.Cofunction.ufl_operands:1: WARNING: py:class reference target not found: FormArgument [ref.class]
/__w/firedrake/firedrake/venv/lib/python3.12/site-packages/firedrake/functionspaceimpl.py:docstring of firedrake.functionspaceimpl.WithGeometryBase.reconstruct:4: WARNING: py:class reference target not found: finat.ufl.FiniteElement [ref.class]
/__w/firedrake/firedrake/venv/lib/python3.12/site-packages/firedrake/interpolation.py:docstring of firedrake.interpolation.Interpolate.ufl_operands:1: WARNING: py:class reference target not found: FormArgument [ref.class]
/__w/firedrake/firedrake/venv/lib/python3.12/site-packages/firedrake/external_operators/abstract_external_operators.py:docstring of firedrake.external_operators.abstract_external_operators.AbstractExternalOperator.ufl_operands:1: WARNING: py:class reference target not found: FormArgument [ref.class]
/__w/firedrake/firedrake/venv/lib/python3.12/site-packages/firedrake/external_operators/ml_operator.py:docstring of firedrake.external_operators.ml_operator.MLOperator.ufl_operands:1: WARNING: py:class reference target not found: FormArgument [ref.class]
/__w/firedrake/firedrake/venv/lib/python3.12/site-packages/firedrake/external_operators/point_expr_operator.py:docstring of firedrake.external_operators.point_expr_operator.PointexprOperator.ufl_operands:1: WARNING: py:class reference target not found: FormArgument [ref.class]
/__w/firedrake/firedrake/venv/lib/python3.12/site-packages/firedrake/ml/jax/ml_operator.py:docstring of firedrake.ml.jax.ml_operator.JaxOperator.ufl_operands:1: WARNING: py:class reference target not found: FormArgument [ref.class]
/__w/firedrake/firedrake/venv/lib/python3.12/site-packages/firedrake/ml/pytorch/ml_operator.py:docstring of firedrake.ml.pytorch.ml_operator.PytorchOperator.ufl_operands:1: WARNING: py:class reference target not found: FormArgument [ref.class]

This is caused by incompatibilities between Sphinx and type hinting (sphinx-doc/sphinx#11225) and was introduced into UFL in https://github.com/FEniCS/ufl/pull/387/files.

When that all gets sorted out we should re-enable Sphinx to resolve the links.