DOC: Add release process to the docs
This was in a branch in the release repo, and not fully updated. I expect to automate some of the tasks shortly, but having the current release process here while things are improved is still helpful.

The final testing before the release is not added here. I think that's the first thing to move to the CI.

CC: @lithomas1 @mroeschke 
Thanks @lithomas1 and @mroeschke, great feedback. All comments should be addressed now. Awesome thanks @datapythonista 