TYP: expand acceptable types for pd.to_datetime()
We document that `pd.to_datetime()` can accept a `DataFrame` or "dict-like" as an argument, but the overloads did not specify that.  This PR adds that.  

In addition, there was no overload for `Index` or `ExtensionArray` as an input type that converts to `DatetimeIndex`

Also, changed the overload for `pd.to_datetime()` on a scalar to only return `Timestamp` to make user code easier to write.  E.g.,
```python
import pandas as pd
ts: pd.Timestamp = pd.to_datetime("2022-03-05")
```
with the current version where `pd.to_datetime()` can return `Timestamp | NaTType`, users would have to do a `cast` 

thanks @Dr-Irv 