Examples are required to be strings

Right now each example is required to be a string, even if the type isn't necessarily a string.

Given this input:

```python
build_on: list[str] | str = pydantic.Field(
min_length=1,
examples=[
"amd64",
["arm64", "riscv64"]
]
)
```

I would expect it to work properly. However, instead I get this exception:

```
Exception occurred:
File "/home/lengau/Work/Code/craft-application/.venv/lib/python3.12/site-packages/yaml/reader.py", line 178, in update_raw
data = self.stream.read(size)
^^^^^^^^^^^^^^^^
AttributeError: 'list' object has no attribute 'read'
The full traceback has been saved in /tmp/sphinx-err-ua2zgx1a.log, if you want to report the issue to the developers.
```

Reproduce the issue with the shown Pydantic Field example and inspect the Sphinx extension's handling of the field's examples before the yaml.reader failure in the traceback. Done means examples containing both strings and lists are accepted without the reported AttributeError and the YAML reference documentation is generated successfully.

Written by the indexing model from the issue text.