Rewrite mixins page

[MaryaBelanger]

Rewrite/rearrange contents on mixins page in a more sensible way, details in these comments: #4834 (comment)

[parlough]

Some more specific feedback on the page from @feinstein:

I think we should expand the mixin docs, there are more details to them that are not in the tutorial.

For example, a mixin can have abstract functions, that should be declared into a class that uses the mixin, so on this sense a mixin can have abstract methods, or getters, etc, even though its not an abstract class mixin. I discovered this by exploring mixin, so I wonder what else I can do with them that I just didn't find out for myself.

Also we should be more emphatic about the mixin limitations, we only show them in the class, mixin, or mixin class? section for a little bit.