Convert readthedocs links for their .org -> .io migration for hosted projects

[adamchainz]

trafficstars

As per their blog post of the 27th April ‘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.

[adamchainz]

The link on the Github repo description also needs updating.

[coveralls]

Coverage remained the same at 92.494% when pulling 04f790507f5a51bce3809fe59d6587190735f6d9 on adamchainz:readthedocs.io into 5c9cf65e9db80c47d3947255dfcf3b314cfe8c25 on jorgebastida:master.