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

[jlopezpena]

Minimally reproducible example:

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)