Document DBaaS API

User story:

User should be able to discover and understand DBaaS API better.

Currently it is very hard to find out where is API, and there is no good examples how to use it, as well as documentation is hidden in "Model" field. Also API is not mentioned in the doc and there is no full example how to use it step by step.

UI/UX:

https://percona-pmm.readme.io/reference/introduction 

Acceptance criteria:

User could read about API in Documentation and create clusters with API and without UI.

Out of scope:

none

Suggested implementation:

 https://percona-pmm.readme.io/reference/introduction

How to test:

Fully functional DBaaS through API in some script.

Details:

none