docs: react-overridable mechanism

[mvidalgarcia]

After having integrated react-overridable a thorough update of documentation needs to be done.

• Eliminate renderElement() bits
• Explain react-overridable concept and how to use it
• Explain how to namespace components (overridableId)
• Expose all <Overridable /> ids to ease overriding components.

[noorfathima11]

If someone is looking for how to eliminate renderElement prop from their code and use react-overridable, here is a small example of how I did for BucketAggregation component.