Update Release Process docs

The CKAN release process is currently documented in at least two places:

https://github.com/okfn/ckan/wiki/Release-Process

http://docs.ckan.org/en/latest/release-cycle.html

The one in the the sphinx docs is kind of ... empty. And I think the one on the wiki has diverged from reality.

We should pick one of these two places, make it complete and up to date, then delete the other one.

@amercader I took the libery of assigning this to you :)