ManimDirective needs a `ref_methods`

[jsonvillanueva]

Some of the current documentation uses :ref_functions: in the example documentation which is supposed to link to the proper function; however, it is often the case that the function is actually a method. In these instances, a ref_methods would be appropriate to have, and the usage of this in the examples should be updated accordingly.

See https://github.com/ManimCommunity/manim/blob/master/docs/source/manim_directive.py#L133-L135