Update rdt urls

[simonrw]

readthedocs are changing their hosting addresses to *.readthedocs.io instead of *.readthedocs.org:

Hello!
Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.

Changes to provide security against broader threats have been in place for a while, however there are still a few scenarios that can only be addressed by migrating to a separate domain.

We implemented session hijacking detection and took precautions to limit cookie usage, but there are still a number of scenarios utilizing XSS and CSRF attacks that we aren't able to protect against while hosting documentation from subdomains on the readthedocs.org domain. Moving documentation hosting to a separate domain will provide more complete isolation between the two user interfaces.

Projects will automatically be redirected, and this redirect will remain in place for the foreseeable future. Still, you should plan on updating links to your documentation after the new domain goes live.

We should change any urls pointing to our documentation to use the new domain.

[simonrw]

$ ag readthedocs.org                                                                                            lordran
astropy_helpers/CHANGES.rst
123:- When the documentation is built on readthedocs.org, MathJax will be

docs/index.rst
22:.. image:: https://readthedocs.org/projects/donuts/badge/?version=latest
23:    :target: http://donuts.readthedocs.org/en/latest/

README.rst
22:.. image:: https://readthedocs.org/projects/donuts/badge/?version=latest
23:    :target: http://donuts.readthedocs.org/en/latest/
32:Project documentation: https://donuts.readthedocs.org/en/latest/

We should not change the ones in astropy_helpers but we need to change our documentation links.