DOC: better document Dtypes docstrings + avoid sphinx warnings
- Use ``class_without_autosummary`` template for the dtypes (similar to what `CategoricalDtype` already did) to avoid a bunch of sphinx warnings
- Therefore, explicitly list some attributes in the docstrings (for now, I went with only documenting the dtype-specific "metadata")
- And while at it, I also gave the docstrings an update (documenting the parameters, etc)
- And for doing the above, turned some metadata attributes of the dtypes into properties, so they 1) can be documented (have a docstring) b) cannot be set by the user (xref https://github.com/pandas-dev/pandas/issues/26096)