Make type hints for `@dlt.source` / `@dlt.resource` decorated functions more prominent in the documentation

### Documentation description

The correct type hints for a function decorated with `@dlt.source` or `@dlt.resource` are difficult to find in public facing docs. Finding a good example required a pretty deep dive into the code base.

I would expect a short section in the source docs, resource docs and cookbook example with a typed example, including import path and annotation.

This would help teams that adopt dlt and strict typing, prevent guesswork and quicken adoption.

### Are you a dlt user?

I'm considering using dlt, but the documentation is unclear.