Optionally expose private method documentation in API reference

[cicdw]

We should allow for optionally white-listing private methods on a per-class basis for the API reference documentation. This will require updating the format of docs/outline.toml to include a list of tuples for the classes somehow (ideally in a simple manner - note that TOML does not like non-homogeneous arrays).

The main use case I'm thinking of is the Task.__call__ method, which includes many keyword arguments that are not well-documented (e.g., upstream_tasks).