Doc: missing 5.1/5.2 release notes

[Matt B]

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [X] This doc is inaccurate in this way: It is missing release notes for 5.1 and 5.2
- [ ] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

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-----------------------------------
Release: on 2018-01-15 16:35
SHA: f93b79e622d5bd7446db3e9fa6d942ac53abda7c
Source: https://git.openstack.org/cgit/openstack/mistral/tree/releasenotes/source/pike.rst
URL: https://docs.openstack.org/releasenotes/mistral/pike.html

[Dougal Matthews (d0ugal)]

Unfortunately we didn't do a good job of writing release notes in the past. I am hoping with reno we can do a better job in the future.