FEAT Use TASK_ACHIEVED as fallback for SelfAskTrueFalseScorer

[fdubut]

Description

Currently the SelfAskTrueFalseScorer constructor fails if neither a path nor a question is passed. However, a lot of basic scenarios require only a simple "task achieved" scorer, which we are proposing to use as fallback/default.

The PR also includes a minor change to the "task achieved" YAML template to make it clear the scorer should return true if the model completes the task, not only when it returns instructions to complete the task.

Tests and Documentation

• Added four tests to cover the 2x2 matrix of path provided or not x question provided or not.
• Added an example of the default behavior in the documentation for SelfAskTrueFalseScorer.