[FR] Make it easier to show source in docs for thin classes

# Minimally reproducible example:

```python
from enum import Enum # StrEnum for python >= 3.12
from nbdev import show_doc

class Colors(str, Enum): # Or `StrEnum` if on python >= 3.12
"Enum with the valid colors we can use in our package"
RED = "red"
GREEN = "green"
BLUE = "blue"

show_doc(Colors)
```

# Issue description

This it not really a bug, more like a feature request. When defining thin classes, such as enums or dataclasses, the documentation for those classes shows a very uninformative `ClassName (*values)`.

For this use case, it would be really useful to have an option that would allow inlining the entire class source in the docs, to get a view of all the possible values. To avoid breaking existing code this could be implemented as an optional argument to the `show_doc` method, so to enable this one would do something like `show_doc(Colors, full_source=True)`

Start at nbdev's show_doc entry point and reproduce the Colors example to inspect how a thin class is currently rendered. Trace the documentation rendering path, then verify that an opt-in full_source=True form shows the complete class source while existing show_doc(Colors) behavior remains unchanged.

Written by the indexing model from the issue text.