Add toggle_implementation to the sphinx documentation

[robrap]

The annotation toggle_implementation should be added to the sphinx documentation, because knowing that a toggle is a “DjangoSetting” vs a “CourseWaffleFlag” gives a lot of information around the capabilities of the toggle and how and where to configure it, once you understand each type of implementation.

See https://discuss.openedx.org/t/which-toggle-annotations-should-be-published/4690

Question:

• Is it enough to capture this decision in the commit comment vs an ADR?