feature: show __init__() signature

[osamimi]

For a standard data class, it seems like mkdocstrings can show the signature of the automatically generated init() method:

It would be great to do that for pydantic classes too, so it's clear to the user how to create an instance of the class.

See mkdocstrings/python#234 (comment)

[danielgafni]

@pawamoy I have another use cases that's even more important: my Pydantic classes can have custom __init__ methods that often coerce a Union of a few similar types into a canonical representation that's stored in the Pydantic field.

Currently these custom constructors are completely missing from docs. But at least overloads are there!