As per [their blog post of the 27th April](https://blog.readthedocs.com/securing-subdomains/) ‘Securing subdomains’:
> 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.
Test Plan: Manually visited all the links I’ve modified.
* Add tarball_bundler example inline
* Add tarball_bundler example to bundler doc too
* Cross-ref from distributing doc to doc about writing extensions
(c) Copyright IBM Corp. 2016
michaelpacer
Adam Chainz
Peter Parente
Daniel McDonald
Min RK
Carol Willing
Nicholas Bollweg