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.

They were getting quite long. They've been split up in several
subsections. Some of them are borrowing heavily from the Django docs.

* Remove profiling middleware from default setup
* Document dependency on libjpeg

To avoid confusion.

Fixes #429

Better than having a static file that contains a single link...