Move Docs to readthedocs.io ?

[Jan van Doorn]

I propose we move our docs to readthedocs - like : 
http://incubator-trafficcontrol.readthedocs.io/en/latest/ (this is setup 
w my clone).

This will allow us to have multiple versions of docs online, do 
automatic updates of the released and latest docs and will get us the 
"Edit on Github" button that Ryan requested ( 
https://github.com/apache/incubator-trafficcontrol/issues/1625 ).


Thoughts?


JvD