Add typing helper classes to auto-generated Reference docs

We don't (yet) have typing helper classes documented in our generated reference docs.

https://sdk.meltano.com/en/latest/reference.html

The classes are defined here:

https://gitlab.com/meltano/sdk/-/blob/main/singer_sdk/typing.py

Docs templates and generators are here:

• https://gitlab.com/meltano/sdk/-/blob/main/docs/reference.rst
• https://gitlab.com/meltano/sdk/-/tree/main/docs/classes