enable readthedocs PR builds and move readthedocs config into .readthedocs.yml

[graingert]

steps that are needed:

go here: ​https://readthedocs.org/dashboard/treq/advanced/#id_external_builds_enabled and tick this box ​

then screenshot the "Default settings" fromset, it should look something like this: I can use that to create a .readthedocs.yml file

then go here ​https://readthedocs.org/dashboard/treq/integrations/create/ to double check the webhook is enabled ​https://i.imgflip.com/4gfv14.jpg