Add API section in docs for React API

[sculpt0r]

Are you reporting a feature request or a bug?

Feature

Provide detailed reproduction steps (if any)

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Expected result

We should have a separate section in API docs for React API, e.g.

Actual result

Currently, all API references are described in the guide section of the documentatnion.

Other details

This issue is created as a result of a review in ckeditor/ckeditor4-docs#355.
Context details and proposal of the solution: ckeditor/ckeditor4-docs#355 (comment)

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[f1ames]

Let's wait with this after we publish stable version. Blocked for now by #229.