Update readthedocs for mixbox

[ikiril01]

The current readthedocs documentation doesn't take into account the mixbox related changes, and so it should be updated accordingly.

[bworrell]

@ikiril01, this should wait until we are ready to push out a mixboxified python-maec release.

[ikiril01]

@bworrell yup, agree 100%! I should have clarified this point in the issue description :)

[gtback]

The "default" in all of the RTD pages should be "stable", not "latest". It's possible we should add a warning in master, which will get picked up if people try to look at the "latest" docs.