Aggregations docs

[andersosthus]

I couldn't find any documentation (in the develop/new-docs branches) for the new Aggregation API.

Have you started working on it, or would you like me to take a stab at it ? Currently the NEST doc site only have Facet information, and I guess the docs should also reflect Aggregations ?

[gmarz]

@andersosthus As I stated in #786, we are currently working on improving the documentation significantly. We have this branch https://github.com/elasticsearch/elasticsearch-net/tree/feature/improve-docs dedicated to updating the docs. Haven't gotten to aggregations yet, but plan to- any contributions are more then welcome!

[Mpdreamz]

closing this after @andersosthus wrote a ton of aggregation docs in PR #792

Thanks again @andersosthus !