Mistral Document Fixes

There are many places where there are minor changes to be made and a few links are to be added. This blueprint is to bring all those changes to the documentation.

Gaps:
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1. Documentation style not maintained properly
2. Some critical links are missing

From Renat:
* Article "Mistral Overview" is outdated. For example, the use cases listed there are not the most common (according to our data). Also the very first paragraph (What is Mistral) and the very last one "Rationale" should be more clear, now they look too abstract. (http://docs.openstack.org/developer/mistral/overview.html)

* Article "Architecture" is too small. It makes sense to add more information. (http://docs.openstack.org/developer/mistral/architecture.html)

* We need to either remove "Roadmap" from the index page completely and add it to some special article devoted to development or at least to move it to the end of the list (http://docs.openstack.org/developer/mistral/)

* Article "Mistral Client / CLI guide" is messed up. Two problems: 1) "Mistral Client" in this name is about installation which doesn't explicitly tell this (should be "Mistral Client Installation Guide") 2) "CLI guide" is not about installation and should exist as a separate article. (http://docs.openstack.org/developer/mistral/)

* I think we need to change "How to write an Action Plugin" to just "How to write a custom action". I think the word "plugin" here is not important from user perspective (http://docs.openstack.org/developer/mistral/)