Building the documentation produces many warnings and some errors

E.g. see [this workflow](https://github.com/FEniCS/ufl/actions/runs/17440496452/job/49521957607):
```
reading sources... [ 96%] releases/v2018.1.0
reading sources... [100%] releases/v2019.1.0
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/action.py:docstring of ufl.action.Action:5: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/classes.py:docstring of ufl.action.Action:5: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/classes.py:docstring of ufl.form.Form.terminal_numbering:5: WARNING: Inline interpreted text or phrase reference start-string without end-string. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/classes.py:docstring of ufl.form.Form.terminal_numbering:5: WARNING: Inline interpreted text or phrase reference start-string without end-string. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/domain.py:docstring of ufl.domain.MeshSequence:12: WARNING: Inline interpreted text or phrase reference start-string without end-string. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/form.py:docstring of ufl.form.Form.terminal_numbering:5: WARNING: Inline interpreted text or phrase reference start-string without end-string. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/form.py:docstring of ufl.form.Form.terminal_numbering:5: WARNING: Inline interpreted text or phrase reference start-string without end-string. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/formoperators.py:docstring of ufl.formoperators.adjoint:7: WARNING: Inline emphasis start-string without end-string. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/operators.py:docstring of ufl.operators.Dx:3: WARNING: Inline emphasis start-string without end-string. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/operators.py:docstring of ufl.operators.div:6: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/operators.py:docstring of ufl.operators.grad:7: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/operators.py:docstring of ufl.operators.nabla_div:6: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/operators.py:docstring of ufl.operators.nabla_grad:7: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/tensors.py:docstring of ufl.tensors.as_scalar:6: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/tensors.py:docstring of ufl.tensors.as_scalars:6: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/__init__.py:docstring of ufl.action.Action:5: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/__init__.py:docstring of ufl.operators.Dx:3: WARNING: Inline emphasis start-string without end-string. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/__init__.py:docstring of ufl.core.external_operator.ExternalOperator:1: WARNING: duplicate object description of ufl.core.external_operator.ExternalOperator, other instance in api-doc/ufl, use :no-index: for one of them
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/__init__.py:docstring of ufl.form.Form.terminal_numbering:5: WARNING: Inline interpreted text or phrase reference start-string without end-string. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/__init__.py:docstring of ufl.form.Form.terminal_numbering:5: WARNING: Inline interpreted text or phrase reference start-string without end-string. [docutils]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/__init__.py:docstring of ufl.core.multiindex.Index:1: WARNING: duplicate object description of ufl.core.multiindex.Index, other instance in api-doc/ufl, use :no-index: for one of them
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/__init__.py:docstring of ufl.core.interpolate.Interpolate:1: WARNING: duplicate object description of ufl.core.interpolate.Interpolate, other instance in api-doc/ufl, use :no-index: for one of them
...
WARNING: more than one target found for cross-reference 'FormArgument': ufl.classes.FormArgument, ufl.core.terminal.FormArgument [ref.python]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/core/expr.py:docstring of ufl.core.expr.Expr.ufl_operands:1: WARNING: more than one target found for cross-reference 'FormArgument': ufl.classes.FormArgument, ufl.core.terminal.FormArgument [ref.python]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/core/external_operator.py:docstring of ufl.core.external_operator.ExternalOperator.ufl_operands:1: WARNING: more than one target found for cross-reference 'FormArgument': ufl.classes.FormArgument, ufl.core.terminal.FormArgument [ref.python]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/core/interpolate.py:docstring of ufl.core.interpolate.Interpolate.ufl_operands:1: WARNING: more than one target found for cross-reference 'FormArgument': ufl.classes.FormArgument, ufl.core.terminal.FormArgument [ref.python]
/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/site-packages/ufl/core/operator.py:docstring of ufl.core.operator.Operator.ufl_operands:1: WARNING: more than one target found for cross-reference 'FormArgument': ufl.classes.FormArgument, ufl.core.terminal.FormArgument [ref.python]
...
```
It would probably be a good idea to build the documentation passing `--fail-on-warning` to make sure that this is caught in future.